The battle for control of the U.S. Senate is close to being a tossup, but with perhaps a very narrow advantage for Democrats, according to the FiveThirtyEight forecast, which launched this week. In fact, control of the chamber could easily be determined by the presidential election because of the possibility of a 50-50 tie in the Senate, which would be broken by the incoming vice president.

Each party entered the cycle with its own set of advantages. For Democrats, the principal asset is opportunity: Of the 34 seats up for grabs this year, 24 are held by Republicans. Furthermore, Republicans last won or defended these seats in the wave election year of 2010, which featured heavy Republican turnout amid dissatisfaction with President Obama’s health care bill and his performance during his first two years in office. The climate is unlikely to be as favorable to the GOP this time around.

By contrast, Republicans’ biggest advantage is the inertia of incumbency — plus the fact that they have some room to spare. Although Congress as a whole remains near record-low levels of popularity, some individual Republican senators, such as Rob Portman of Ohio and Chuck Grassley of Iowa, remain relatively popular. Meanwhile, only two Republican incumbents, Dan Coats of Indiana and David Vitter of Louisiana, are retiring from the Senate, and none lost in a primary challenge. That cuts down on the number of targets for Democrats, who have to pick up somewhere between four and six seats to regain control of the chamber, depending on whether they hold on to one of their seats, in Nevada, and whether they win the presidency — itself an increasingly uncertain prospect.

All told, Democrats have a 59 percent chance of controlling the Senate in 2017, according to FiveThirtyEight’s polls-plus forecast, the default version of our newly relaunched Senate model, which is based on a combination of state-by-state polling and “fundamental” factors, such as the partisanship of each state. Democrats’ odds are the same, 59 percent, based on our polls-only forecast, which is a new feature this year and, as its name implies, is based on polling alone in states with a sufficient number of surveys. These forecasts include about a 15 percent chance that the Senate will be split exactly 50-50. In these cases, our models draw from our presidential election forecasts to estimate the chance that Tim Kaine or Mike Pence will be the new vice president and give his party Senate control.

To see why the race is relatively even, we’ll take a whirlwind tour of Democrats’ most plausible pickups. You may have heard elsewhere that the Democrats’ magic number is four seats. But that assumes they hold onto all their seats and win the presidency, and as I hinted at above, that’s not a safe assumption; instead, they’ll require somewhere between four and six seats. Their first problem is Nevada, where they’re defending the seat held by retiring Senate Minority Leader Harry Reid. In theory, Nevada should lean slightly blue in a presidential year, but it hasn’t done so in either the presidential or the state’s Senate race so far; recent polls there show the Republican Senate candidate, U.S. Rep. Joe Heck, slightly ahead of Democrat Catherine Cortez Masto, a former Nevada attorney general. Thus, Nevada is, at best, a tossup for Democrats, meaning that they might require five Republican gains instead of four.

Democrats will require yet another seat if Donald Trump wins the presidential election instead of Hillary Clinton, potentially raising the bar to six seats. While it’s somewhat far-fetched to think that Democrats could reclaim the Senate in the same year that Trump wins the presidency, it’s not out of the question. There are indications from voters of more willingness to split tickets than in recent past elections, with these splits benefiting either party in different states. (Jason Kander, the Democratic Senate candidate in Missouri, for instance, is running ahead of Clinton there.) Furthermore, a Trump win would probably be narrow and could conceivably be caused by defections of Democratic-leaning voters from Clinton to third-party candidates who might still vote Democratic down-ballot.

Below are Democrats’ pickup opportunities in order of likelihood, with percentages taken from the polls-plus version of our forecast.

Likely Democratic pickup

This is the only possible pickup that seems close to a sure thing for Democrats. Former Democratic Sen. Russ Feingold is in a rematch with Republican incumbent Sen. Ron Johnson, whom Feingold lost to in 2010 despite having relatively good approval ratings. Polls have consistently shown Feingold ahead this year, and he stands to benefit from Wisconsin’s traditionally high presidential-year turnout.

Probable Democratic pickups

Both Illinois and Indiana are underpolled, although the states are otherwise very different types of pickup opportunities for Democrats. Illinois is blue enough that even the expressly moderate Republican Sen. Mark Kirk, who has openly denounced Trump, is swimming against a partisan tide. Polls there show Democrat Tammy Duckworth ahead, although there has been only a handful of nonpartisan surveys.

In Indiana, by contrast, former Democratic Sen. Evan Bayh is ahead in polls, but such a result would cut against the state’s partisan environment, which is solidly Republican in the presidential race. Also, the size of Bayh’s lead varies based on the sponsor of the poll, with single-digit leads over U.S. Rep. Todd Young in nonpartisan surveys but larger leads in polls sponsored by Democratic-leaning groups. The polls-plus forecast in Indiana, as cited above, gives Bayh a 75 percent chance, although polls-only is considerably more confident in him, putting his chances above 90 percent.

Tossups

If everything else is going well for Democrats — they win the presidency, Nevada and all three of the races above — they’ll need only one of these two seats. Clinton’s polling in Pennsylvania and New Hampshire has held up relatively well even as it has deteriorated elsewhere, but the incumbent Republican senators in both states are running ahead of Trump. In Pennsylvania, Democrats have an inexperienced candidate in Katie McGinty, who has not previously held elected office. But she’s held narrow leads over Republican incumbent Pat Toomey in the majority of recent polls, and Toomey may be too conservative for the state. In New Hampshire, Republican incumbent Kelly Ayotte is holding on for her political life against a tough opponent, Gov. Maggie Hassan, while trying to find the appropriate level of distance from Trump.

Plausible Democratic pickups

If the five races I mentioned above were Democrats’ only opportunities, they’d be underdogs to reclaim the Senate. But they have some backup options. In Missouri, first-term incumbent Roy Blunt has long had middling approval ratings, and Democrats may have a talented enough candidate in Kander, the state’s secretary of state, to make up for Missouri’s increasingly Republican lean at the top of the ticket. North Carolina also features an underwhelming Republican incumbent in Sen. Richard Burr, but the circumstances there are otherwise somewhat the opposite of those in Missouri: Democrats don’t have a top-tier candidate (their nominee is former state Rep. Deborah Ross), but the state is becoming bluer. Ross has gained in recent polls, and the state is on the verge of becoming a tossup.

Florida is in something of its own category, with Marco Rubio having decided to run for re-election at the last minute after having flamed out of the GOP presidential primary. The presidential run damaged Rubio’s approval rating at home, but he’s running against a Democratic opponent in Patrick Murphy who’s also somewhat damaged goods, having been caught padding his résumé, among other minor scandals. Murphy has raised an impressive amount of money, however, and will have a chance to close his current, roughly 5-point deficit in the polls.

Long-shot opportunities

These states will probably only come into play for Democrats in the event of a late-breaking national wave in their favor. Still, each represents a lottery ticket of a sort — the chance to benefit from a last-minute scandal or gaffe.

Some of them are also interesting on their own merits. Kentucky, for instance, theoretically brings vulnerability for incumbent Sen. Rand Paul, whose image at home suffered during his failed presidential bid. But Democrats have failed to knock off unpopular Republicans in Kentucky in the past, and their candidate this year — Lexington Mayor Jim Gray — is a long shot.

Louisiana holds what’s technically a primary election on Nov. 8, with multiple candidates from both parties appearing on the ballot and the top two finishers advancing to a Dec. 10 runoff unless someone gets a majority of the vote. With 24 candidates on the Nov. 8 ballot — about a half-dozen of whom are regularly tracked in polls — there is the possibility of a crazy outcome, such as Democrats finishing in the top two positions because the Republican vote is divided among several candidates. The Republican candidates include former Ku Klux Klansman David Duke, although he has only about 5 percent of the vote in polls. The probability listed by our model in Louisiana, 14 percent, reflects the chance that the Democrats will eventually control the seat, accounting for the dynamics of both the primary and the runoff.

There’s also one race that Democrats should be unhappy about seeing so far down on this list. That’s the one in Ohio, where former Gov. Ted Strickland was supposed to be competitive but where Republican incumbent Rob Portman has jumped out to a double-digit lead in recent polls.

As a reminder, before you get too carried away with individual races, however, the Senate map can and sometimes does break late. In each of the past five election cycles, one party won all or almost all of the Senate races that would have been considered highly competitive at this point in time. (Democrats did so in 2006, 2008 and 2012; Republicans did in 2010 and 2014.) Our model accounts for the possibility of a wave by noting that outcomes and polling errors are somewhat correlated from state to state.

Thus, while the most likely range of outcomes is a net gain of two to six seats for Democrats, there’s an outside chance — about 15 percent — that they’ll pick up at least eight instead. The magnitude of potential Democratic gains is important because the 2018 Senate map is every bit as bad for Democrats as 2016 is good for them, probably necessitating a cushion of a couple of seats to hold the chamber even if they win it this year.





FiveThirtyEight: Election Update – Sept. 21, 2016