The GOP's Senate Chances Are Looking Grim

The 2016 presidential race has been so bizarre and unprecedented that this year's congressional elections, normally a hot topic in the political press, have received relatively sparse coverage. But while everyone has been fixated on Donald Trump's tweets and Hillary Clinton's emails, a storm has been brewing a bit further down the ballot, and it's now more likely than not that Republicans will lose the Senate in 2016.

Democrats need to flip four or five* GOP-held seats in order to win back control of the Senate. The Huffington Post's forecast model gives Democrats a 78 percent chance of doing this, and Sam Wang at the (highly underrated) Princeton Election Consortium finds that the most likely outcome is that Democrats come out of the election holding 51 seats. There are 100 senators total.

A lot of this is due to structural reasons. The 2010 election was great news for the GOP, as a whole slew of new Republicans were elected to the Senate. But senators face reelection every six years, which means there are disproportionately more Republican senators than Democratic ones up for reelection this year. That fact itself puts the GOP at a distinct disadvantage in 2016, one which was evident the day the 2010 results were announced.

But that's really only half the story. The other half of the story — as always in 2016 — is Trump. By all indications, the GOP candidate's sputtering presidential campaign has been a serious drag on Republican Senate candidates.

During the first two weeks of August, after both parties had held their national conventions, Trump took a nosedive in state polls, and appears to have brought several GOP Senators down with him. In eight states with competitive Senate races, Trump's standing fell by an average of 3.3 points during that time, according to an analysis at FiveThirtyEight. Republicans running for the senate in those same states saw their numbers fall by 2.8 percent over the same period of time.

That may not seem like much. But in a competitive race, a few percentage points is enough to tip it. Pennsylvania Sen. Pat Toomey, for example, went from a two-point lead on Aug. 3 to a two-point deficit by Aug. 15. Sen. Kelly Ayotte suffered an even more precipitous decline: Her two-point lead in the New Hampshire Senate race evaporated in August, and now she's trailing her Democratic opponent by five points.

Some of these Republicans have attempted to distance themselves from Trump, and yet — with the notable exception of Sen. Rob Portman in Ohio — it's not working. One reason for this may be that split-ticket voting has become relatively rare in recent years: In a presidential election, voters are much less likely than they were several decades ago to vote for a Democratic presidential candidate but a Republican Senator, or vice-versa. Instead, they tend to vote the party line.

What this means is that when the GOP nominates a historically-unpopular presidential candidate, one who ends up with an 88 percent chance of losing in late-August of the election year, the Republican Senate candidates end up suffering just as much as he does.

*In order to hold 51 seats in the chamber, Democrats have to win five seats in November. However, if Hillary Clinton is elected president, Tim Kaine would deliver the tie-breaking vote in the Senate pursuant to his role as vice president. In that scenario, Democrats would only need to win four additional seats.