2018 Midterm Election Odds: Using FiveThirtyEight’s Senate Model to Find Betting Value

Sep 14, 2018 5:04 PM EDT

Jul 28, 2016; Philadelphia, PA, USA; Sen. Claire McCaskill, D-MO, speaks during the 2016 Democratic National Convention at Wells Fargo Center. Mandatory Credit: Robert Deutsch-USA TODAY NETWORK

  • The 2018 midterms will take place on Nov. 6, 2018. Just 35 of the 100 Senate seats will be up for grabs, and most of those have Democratic incumbents.
  • ABC News' politics site FiveThirtyEight released its Senate model, which currently gives Republicans a 67.3% chance to retain control of the Senate.
  • Politics betting site PredictIt has odds for specific Senate races, and comparing their odds to FiveThirtyEight's model can point out potential betting value.

Nate Silver and Co. at ABC News’ FiveThirtyEight.com have released their model predicting the 2018 Senate race in the November midterms.

Unlike the House of Representatives, in which all 435 seats are up for grabs this fall, only 35 of the 100 total Senate seats are up for election.

The Democrats have to defend many more seats than the Republicans, so although FiveThirtyEight is predicting the Dems to grab 25 of the 35 available seats, the odds are still high that the Republicans retain control of the Senate.

FiveThirtyEight has the overall Senate majority odds at 67.3% for the GOP, which is about in line with a couple betting markets:

Odds Republicans Will Retain Control of Senate in 2018

PredictIt, a politics betting market, not only has odds on which party will have a majority after the midterms, but it also allows you to wager on a variety of specific Senate races — 24 of the 35, to be exact.

FiveThirtyEight predicts each race, so we can compare the lines from the two sites to see if there’s any betting value.

PredictIt Prices vs. FiveThirtyEight Odds for 2018 Senate Races

The best values, according to the FiveThirtyEight Senate model, appear to be with the red-state Democrats such as Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota, Joe Manchin of West Virginia and Claire McCaskill, pictured above, of Missouri.

PredictIt has those races as either toss-ups or with the Republican favored, whereas FiveThirtyEight is more bullish on the Democrats’ chances. Donnelly is the best value on the board: FiveThirtyEight has him at 76.8% to retain his seat, whereas PredictIt has it as a 50/50 proposition.

It’s possible FiveThirtyEight is more heavily accounting for the current political climate, which definitely favors the Democrats.

According to RealClearPolitics.com, Democrats currently have a +8.3 spread in 2018 generic congressional polling. Per Nate Silver, they’d need to get much higher than that to have a shot at actually swinging the majority to their side. But still, the climate is ripe for Democrats to make it a contested race.

Make sure to monitor the model and how it changes over the coming months. There are few reliable polls for individual Senate races, so the ones that do get released will certainly shift odds quickly.

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