UPDATE: State of play in the 2018 Senate elections

Just as FiveThirtyEight.com released its Senate forecast, I update this post on the outlook for Democrats in the 36 elections for the United States Senate (“Senate”) this November 6 (and beyond, in the Mississippi special election). Feel free to compare and contrast the two.

To be more precise, I am updating the tables and a few paragraphs of text to reflect the following changes:

1. FiveThirtyEight.com now projects Democrats to win the total vote of the United States House of Representatives (“House”) by 9.0 percentage points.
2. Since Labor Day, a slew of new polls have been released.
3. I corrected a flaw in how I weighted time and pollster ratings in the “adjusted pollster averages.”
4. I now weight my adjusted polling average three times more than “fundamentals.”
5. For the four Senate races in Table 4 and three Senate races listed as “Wildcards,” I now use the simple average margin in all polls released in 2018, as listed on that election’s Wikipedia page.
6. I revised how I calculate incumbency advantage.

Just bear with me while I describe my Senate incumbency advantage calculation.

For each Senate election in 2012, 2014 and 2016, I calculated an “expected” outcome by adding the state’s partisan lean (3W-RDM) to the margin by which Democrats topped (or fell) to Republicans in the total national vote for Senate that year (D+0.9, D-5.8, D+12.1 percentage points in 2016, 2014, 2012, respectively). Next, I subtracted that from the actual Democratic margin in each race. I then calculated, for each election year, the average of these differences for the Senate races in which no incumbent sought reelection; open seats elections exemplify “generic Democrat” versus “generic Republican” elections. Next, I averaged the difference between each Democratic, and each Republican, incumbent’s “actual-minus-expected” margin and the open seat average for each of 2012, 2014, 2016. Finally, I took the simple average of these “incumbent-minus-open” differences, separately for Democratic and Republican incumbents, for each of the three year.

Using this new method, on average Democratic incumbency still adds 8.3 percentage points while Republican incumbency adds 9.6 percentage points.

Here is an example using 2016 data. On average, in the five open seat Senate elections that year, the Democratic-minus-Republican margin was 3.9 percentage points higher than would be expected based solely on partisan lean plus 0.9.  The actual margin for the seven Democratic incumbents averaged 10.4 percentage points (or 6.5 percentage points higher than the open seat average), and the actual margin for the 22 Republican incumbents averaged 7.5 percentage points (or 11.4 percentage points higher than the open seat average). Thus, in 2016 the Democratic and Republican incumbency advantages were 6.5 and 11.4 percentage points, respectively. The Democratic incumbency advantages in 2014 and 2012 were 9.8 and 8.5 percentage points, respectively, while for Republican incumbents the values were 1.1 and 16.5 points, respectively.

With that, here are the updated tables:

Table 1: Summary of 2018 Polling Data in 10 Key 2018 Senate Elections

Table 2: Most-endangered 2018 Democratic Senate incumbents

 Table 3: Most-endangered 2018 Republican-held Senate seats

Table 4: Once-endangered, now safe 2018 Democratic Senate seats

Wildcards. New Jersey Democratic Senator Bob Menendez “should” be ahead of businessman Bob Hugin by 29.3 percentage points. However, Menendez “only” leads by an average of 10.0 percentage points in six 2018 polls—and just 6.0 percentage points in three July/August polls. The weighted average of these two percentages is 14.8 (11.8 using the three most recent polls), which is about where the race should ultimately land.

Minnesota Democratic Senator Tina Smith “should” be leading Republican State Senator Karin Housley by 14.6 percentage points; as an appointed senator, I arbitrarily cut her incumbency advantage in half. However, in an average of four polls released in 2018, Smith is ahead by “just” 8.4 percentage points; I currently anticipate Smith winning reelection by around the weighted average of 10.0 percentage points.

Appointed Mississippi Senator Cindy Hyde-Smith is very likely to face former Agriculture Secretary Mike Espy, a Democrat, in a November 27 runoff. Hyde-Smith would be expected to prevail over a generic Democrat by 14.2 percentage points, though in four head-to-head polls with Espy, she leads by “only” 7.3 percentage points (and 5.5 in the two most recent polls). The weighted averages suggest Hyde-Smith will prevail by 7.7-9.0 percentage points—though if this race ends up being decisive for Senate control, anything is possible.

The 2018 Florida Senate election between incumbent Democrat Bill Nelson (left) and Republican Governor Rick Scott is by far the most polled of all Senate races, and it could easily be the most expensive—and decisive for Senate control.

Bottom line. The likeliest outcomes are still between Democrats losing a net of two seats (flip Arizona; lose Nevada, North Dakota, Florida, Missouri) and gaining a net of one seat (flip Arizona, Nevada; lose North Dakota) with Tennessee and Texas JUST out of reach for Democrats. Still, there is a path for Democrats to recapture the Senate by starting with the D+1 seat outcome and winning any one of North Dakota, Tennessee and Texas.

Until next time…