I will now do the same for the U.S. Senate (Senate), which Republicans now control 52-48 (including two Independents who caucus with the Democrats).
Just bear with me while I review some recent electoral history.
In 2006, the second midterm election of the George W. Bush Administration, Democrats won 53.2% of all Senate votes cast (compared to 41.8% for Republicans and 5.0% for Independent and third-party candidates), netting six seats in 2006 and a 51-49 majority. This majority rested in part on victories by Independents Joe Lieberman of Connecticut and Bernie Sanders of Vermont, who subsequently caucused with the Democrats. Lieberman, seeking a 4th term, had lost the Democratic primary to Ned Lamont, then run and won as an Independent. Sanders replaced retiring Independent Jim Jeffords, who had himself caucused with the Democrats.
Six years later, as President Barack Obama was winning reelection by 3.9 percentage points, Democrats won a nearly identical 53.4% of Senate votes (41.8% Republican, 4.8% Independent/third-party), adding two seats for a 55-45 majority. Lieberman did not run for reelection; Democrat Chris Murphy won the open Senate seat. Sanders sought and won reelection. Republican Senator Olympia Snowe of Maine also chose not to seek reelection, and Independent Angus King won the open seat, caucusing with the Democrats.
After a disastrous 2014 election (net loss of nine Senate seats, lost national Senate vote by 7.5 percentage points) and a mediocre 2016 election (net gain of two Senate seats, won by 10.2 percentage points), Democrats currently hold 48 Senate seats (counting King and Sanders). Assuming there will still be a Republican Vice President, Democrats would need to net three Senate seats in the 2018 midterm elections to regain the majority.
Sounds easy right?
In fact, Democrats will be lucky not to suffer a net LOSS of seats.
Of the 33 Senate seats up for election in 2018, 20 are held by Democrats (plus King and Sanders) who first won election to a full term in either 2006 or 2012. Five other Democrats who first won their Senate seat in 1992 or 2000 are also potentially up for reelection, making a total of 25 seats being defended by Democrats in 2018.By contrast, Republicans are only defending eight Senate seats in 2018.
Table 1: U.S. Senate Seats up for Election in 2018
|Current Senator||State||Run 2018?||3W-RDM||2012 Margin||Ave. Pre-2012 Margin (# Races)|
|Mazie Hirono||HI||Yes||34.3||25.2%||N/A (0)|
|Bernie Sanders||VT||Yes||27.7||46.2%||33.1% (1)|
|Dianne Feinstein||CA||Yes||23.2||25.0%||15.5% (4)|
|Ben Cardin||MD||Yes||22.6||29.7%||10.0% (1)|
|Elizabeth Warren||MA||Yes||22.1||7.6%||N/A (0)|
|Kirsten Gillibrand||NY||Yes||21.6||45.9%||27.8% (1)|
|Sheldon Whitehouse||RI||Yes||18.0||29.8%||7.0% (1)|
|Chris Murphy||CT||Yes||12.8||11.8%||N/A (0)|
|Tom Carper||DE||?||12.5||37.5%||25.7% (2)|
|Maria Cantwell||WA||Yes||12.1||20.9%||8.5% (2)|
|Bob Menendez||NJ||Yes||12.0||19.5%||9.0% (1)|
|Martin Heinrich||NM||Yes||6.5||5.7%||N/A (0)|
|Angus King||ME||Yes||5.9||22.2%||N/A (0)|
|Debbie Stabenow||MI||Yes||2.2||20.8%||8.6% (2)|
|Amy Klobuchar||MN||Yes||1.5||34.7%||20.1% (1)|
|Tim Kaine||VA||Yes||1.5||5.9%||N/A (0)|
|Tammy Baldwin||WI||?||0.7||5.6%||N/A (0)|
|Bob Casey||PA||Yes||-0.4||9.1%||17.4% (1)|
|Bill Nelson||FL||Yes||-3.4||13.0%||13.5% (2)|
|Sherrod Brown||OH||Yes||-5.8||6.0%||13.3% (1)|
|Claire McCaskill||MO||Yes||-15.9||15.7%||2.3% (1)|
|Joe Donnelly||IN||Yes||-16.3||5.7%||N/A (0)|
|Jon Tester||MT||Yes||-18.6||3.7%||0.9% (1)|
|Heidi Heitkamp||ND||Yes||-29.4||0.9%||N/A (0)|
|Joe Manchin||WV||Yes||-35.5||24.1%||10.1% (1)|
|Dean Heller||NV||Yes||2.0||1.2%||Appointed 2011|
|Jeff Flake||AZ||?||-9.7||3.0%||N/A (0)|
|Ted Cruz||TX||Yes||-15.3||15.8%||N/A (0)|
|Roger Wicker||MS||Yes||-18.5||16.6%||9.9% (1*)|
|Deb Fischer||NE||Yes||-25.8||15.5%||N/A (0)|
|Bob Corker||TN||Yes||-25.8||34.5%||2.7% (1)|
|Orrin Hatch||UT||No||-33.1||35.3%||27.8% (6)|
|John Barrasso||WY||Yes||-45.7||54.0%||46.8% (1†)|
* Wicker won a special election in 2010 to fill Republican Trent Lott’s seat after he was forced to resign.
† Barrasso won a special election in 2008 after the death of Republican Craig Thomas
In other words, in order for the Democrats to recapture the Senate in 2018, they would have to win three of eight currently Republican seats (37.5%) while not losing ANY of their 25 seats.
However, as shown in Table 1 (data from here, here and here), five Democrats up for reelection represent solidly Republican states, based upon my 3W-RDM measure of how much more or less Democratic a state’s presidential vote has been than the national presidential vote over the last three elections (2008, 2012, 2016), weighted for recency. For example, Claire McCaskill will be defending her seat in Missouri, a state you would expect a Republican presidential candidate to win by 15.9 percentage points if the national presidential vote were dead even
Table 1 also shows whether a Senator will seek reelection in 2018 (or has not made her/his intent clear), her/his 2012 margin of victory and her/his average margin of victory in any previous Senate elections.
In the rest of this post, I will briefly analyze all 33 Senate seats up for election in 2018, organized by party and from most to least vulnerable. This analysis is based solely upon the partisan lean of the state and previous winning margins, ignoring possible partisan “waves” and opposition candidate quality (or lack thereof). I would observe, however, that in the last seven first midterm elections for a new Administration (dating back to 1970), the party holding the White House has lost an average of three Senate seats, with a median loss of two seats.
Republicans. Only one Republican seeking reelection in 2018 represents a Democratic-leaning state: Dean Heller of Nevada (+2.0). Nevada has been trending Democratic and Heller only won his 2012 election by 1.2 percentage points, following his 2011 appointment (after Republican Senator John Ensign resigned). Still, Heller held his seat even as Republicans were LOSING the national Senate vote by 11.6 percentage points, and while Obama won the state by 6.7 percentage points.
Arizona Republican Jeff Flake won his 2012 Senate election by just 3.0 percentage points (again, as Republicans were losing badly nationally), and while Arizona still leans Republican (-9.7), it too has been trending Democratic, along with the rest of the southwest.
If Heller and Flake both lose (and every Democrat and Independent wins), Democrats would still need to flip one more Republican seat to regain control of the Senate. Of the six remaining Republican seats, only two appear even remotely winnable for the Democrats: Texas (Ted Cruz) and Tennessee (Bob Corker).
Texas (-15.3) remains a solidly Republican state amid subtle signs of a pro-Democratic shift (see Arizona above), and Cruz won election in 2012 by nearly 16 percentage points. Nonetheless, early polling suggests that Cruz may be vulnerable. Tennessee is strongly Republican (-25.8), and Corker won his 2012 reelection by a resounding 34.5 percentage points. Corker, however, only won his 2006 race by 2.7 percentage points over Democrat Harold Ford, albeit in a year when Democrats dominated nearly everywhere else.
Finally, it is hard to see Democrats unseating incumbent Republican Senators in Mississippi (-18.5), Nebraska (-25.8) or Wyoming (-45.7!) or winning an open seat in Utah (-33.1), where seven-term veteran Orrin Hatch, currently third in the presidential line of succession (as president pro tempore of the Senate) is retiring.
Democrats. Five Democratic Senators appear to be especially vulnerable in 2018: McCaskill, Joe Donnelly (Indiana), Jon Tester (Montana), Heidi Heitkamp (North Dakota) and Joe Manchin (West Virginia). The average pro-Republican lean of these states is 23.1 percentage points. Donnelly and Heitkamp won their 2012 Senate races by an average of just 3.3 percentage points, riding the strong Democratic wave that year. The good news for McCaskill, Tester and Manchin, however, is that each won reelection in 2012 by a larger margin than in 2006 (McCaskill, Tester) or in 2010 (Manchin). Indeed, McCaskill and Manchin won reelection by 15.7 and 24.1 percentage points, respectively. And Montana’s recent special election for an at-large U.S. House seat, in which the margin increased from the previous election (2016) by 9.6 percentage points pro-Democratic, may be a good sign for Tester.
Five swing-state Democratic Senators would be vulnerable if the partisan winds shifted towards the Republicans, but would be favored to win reelection otherwise: Tim Kaine (Virginia), Tammy Baldwin (Wisconsin), Bob Casey (Pennsylvania), Bill Nelson (Florida) and Sherrod Brown (Ohio). Kaine, Clinton’s 2016 running mate, and Baldwin (who has not announced her 2018 plans) represent narrowly Democratic-leaning states (+1.5 and +0.7, respectively), although their 2012 margins were well below the 11.6 percentage points by which Democrats were winning nationally. As a native Pennsylvanian, I know the Casey name remains revered in the Keystone State. Still, Casey won reelection in 2012 by half the margin by which he unseated Republican Rick Santorum in 2006, and Pennsylvania (-0.4) has been trending Republican. Nelson has represented the Republican-leaning Florida (-3.4) since 2001, winning by an average of just over 13 percentage points. Brown, finally, represents Ohio, a state that has been trending sharply Republican (-5.8), and he only won reelection in 2012 by 6.0 percentage points, more than half the 13.3 percentage points by which he unseated Republican Mike DeWine in 2006. Brown is easily the most potentially vulnerable of these five Democratic Senators, meaning that if he wins reelection in 2018 and Democrats also win back the governor’s mansion in Ohio, he would vault to the top of every Democratic Vice Presidential (and maybe even Presidential) list for 2020.
Ten Democratic Senators and one Independent Senator (Sanders) up for reelection in 2018 represent states that are at least 12 percentage points more Democratic (at the presidential level) than the nation as a whole, and all but two of them Elizabeth Warren (Massachusetts) and Murphy won their 2012 races by at least 19.5 percentage points. Tom Carper of Delaware (+12.5) has not made his 2018 plans clear, but the Democrat would likely have an advantage. Bob Menendez of New Jersey has faced some ethical problems, which could be an issue.
Three additional Democrats (Martin Heinrich [New Mexico], Amy Klobuchar [Minnesota], Debbie Stabenow [Michigan]) and one Independent (King) represent Democratic-leaning states and/or won their 2012 elections by at least 20 percentage points. That makes 15 Democrat/Independent Senators (of 25) who appear at this point to be VERY strong favorites to win reelection (or for a Democrat to retain the seat).
Bottom line. The best case scenario for Democrats is that Democrats successfully defend all 25 seats currently held by Democrats and Independents, while defeating Heller, Flake and Cruz and/or Corker, for a net gain of 3-4 seats, recapturing the Senate. This is within the realm of possibility, but EXTREMELY unlikely.
The worst case scenario for Democrats is that Republicans successfully defend all eight seats, and not only do the five most vulnerable Democrats (McCaskill, Donnelly, Tester, Heitkamp, Manchin) lose, but as many as five of the next-most-vulnerable Democrats (Kaine, Baldwin [or whoever Democrats nominate, if she chooses not to run], Casey, Nelson, Brown) also lose, for a net loss of 5-10 seats. The Republicans need eight seats to gain a filibuster-proof majority of 60 seats. This, too, is within the realm of possibility, but also (if slightly less so) extremely unlikely.
The most likely outcomes lie between a 1-2 seat gain (Democrats lose no more than one seat AND Heller and/or Flake lose) and a five seat loss (Republicans hold all eight of their seats AND McCaskill, Donnelly, Tester, Heitkamp, and Manchin all lose).
Here is the thing, though. Republicans will be defending 22 Senate seats in 2020, a presidential election year. So long as Democrats do no worse than a net loss of 1-2 seats, the likelihood of recapturing the Senate in 2020 would remain high—especially if they recapture the White House as well, requiring only 50 seats for a majority (as the new Democratic Vice President would break the tie).
Until next time…
 The margin drops to 5.9 percentage points if you exclude votes cast for Loretta Sanchez in California. Sanchez, a Democrat, had finished second to Democrat Kamala Harris in California’s all-candidate Senate primary on June 7, 2016.
 This includes Kirsten Gillibrand of New York and Joe Manchin of West Virginia. Gillibrand was appointed to the Senate in 2009 to replace Democrat Hillary Clinton after she became Secretary of State, then won a special election in 2010 and the seat’s regular election in 2012. Manchin won a special election in 2010 to replace the late Robert Byrd, a Democrat, winning the regular election for that seat in 2012.
 As Harry Enten (and others) has observed, Senate elections are behaving more and more like Presidential elections, making the 3W-RDM a good proxy for the partisan lean of a state in Senate elections as well.
 I am also, for now, ignoring two potential “wild cards” in Alabama and Maine. Republican Luther Strange, the former Alabama state attorney general, was appointed to the seat held by Republican Senator Jeff Sessions after he resigned to become Attorney General. While Alabama (-28.4) is a strongly Republican state, the state Republican Party is reeling from the resignation of former governor Robert Bentley following a sex scandal as well as other scandals involving the former Speaker of the (state) House and the former Chief Justice of the State Supreme Court, both Republicans. The special election will be held later this year, and Republicans are already lining up to challenge Strange in the primary. Maine Republican Senator Susan Collins has floated the possibility of running for governor in 2018. Were she to win (a better-than-even chance, I think, should she run), she would appoint a Republican successor to her seat, and that person would face a special election no later than 2020. Appointed Senators face longer odds of winning election in their own right, which would make this a strong pickup opportunity for Democrats sometime after 2018.
 In Senate races, anyway, as Obama lost these five states in 2012 by an average of 19.8 percentage points!