2018 Midterm Election Odds: Republicans Large Favorites to Retain Senate Majority

The 2018 midterm elections are just a little more than a month away, so let's take a look at how a variety of betting and prediction markets peg the upcoming Senate races.

Some context: It's a brutal map this election cycle for Democrats, who will have only 23 seats that are not up for election. Republicans, on the other hand, will have 42 that are not up for election, which certainly gives them a sizable advantage.

Here's a graphic courtesy of FiveThirtyEight illustrating the disparity.

In order to win a true majority (51 of the 100 seats), Democrats would need to win 80% of the races — so 28 of the 35 available seats. They are currently defending 26, which means they'll have to flip two and retain the rest.

If the Democrats win only 27, there will be no majority, which stills favors the Republicans since the GOP has control of the White House — any tie vote is broken by Vice President Mike Pence.

Thus, as you might expect, Republicans are large favorites to retain control of the Senate in November.

Odds to Retain Control of the Senate in the 2018 Midterms

Republicans have a 62.3% implied probability (-165 odds) to retain control, Democrats have a 20.0% (+400) implied probability, while there's a 28.6% (+250) implied probability that there's no majority.

Another betting site, Smarkets, is right in line with those odds. But predictions marketplace PredictIt and FiveThirtyEight's Senate model are more bullish on the Republicans' odds, pegging them at 76.0% and 76.5%, respectively.

It's hard to overstate just how difficult of an election cycle this is for the Democrats, who are currently massive favorites to win back the House of Representatives in November.

The opposition party historically does well in midterms, and the generic ballot shows more support for Democrats at large in the U.S. But because 42 Republican seats are already safe, the math just isn't in their favor this cycle.

Compare that to 2020, when 21 current Republican seats will be up for election versus only 11 for Democrats. Senate terms are six years in length, and a third are up for grabs every two years. This just happens to be the cycle in which Republicans have the safest map and as a result are large favorites to retain control.