Senate Majority Leader Mitch McConnell said Wednesday that Republicans won’t hold hearings on Judge Merrick Garland, President Obama’s nominee to replace Antonin Scalia on the Supreme Court. If McConnell follows through and makes everyone wait until next year, he and his party will be taking a serious risk. The political situation for Republicans could get a lot worse — and McConnell might no longer be in charge of the Senate.

The logic of this isn’t that complicated. Right now, the most likely person to become the next president is Hillary Clinton, who is on the verge of wrapping up the Democratic nomination. The second most likely person is Donald Trump. His path to the Republican nomination is more tenuous than Clinton’s path to the Democratic one. But Trump had a successful day of elections on Tuesday — and if it isn’t obvious how Trump will get to 1,237 delegates, it’s even less obvious how anyone else will become the Republican nominee.

Neither choice is particularly palatable to members of the conservative Senate majority, only one of whom (Jeff Sessions of Alabama) has endorsed Trump. But if their choice between Clinton and Trump is bad enough, Republicans have some further bad news: Democrats have a shot at winning back the Senate. They’ll need to gain four seats to do so if Democrats hold the presidency, or five if a Republican wins. That isn’t a trivial task, but Republicans are vulnerable because a number of their blue- and purple-state senators who won election in the Republican wave year of 2010 are now on the ballot again. Furthermore, Trump could have a negative effect on down-ballot races; so could Ted Cruz, or someone nominated after a contested convention. Although I wouldn’t call Democrats favorites to win back the Senate, a Democratic Senate is probably more likely than not conditional upon Clinton becoming the next president.

If you combine the view of betting markets with a bit of back-of-the-envelope math, it suggests that Republicans face these rough probabilities:

A 40 percent chance of President Clinton with a Democratic Senate.

A 30 percent chance of Clinton with a Republican Senate.

A 20 percent chance of President Trump (probably with a Republican Senate).

A 10 percent chance of Cruz, John Kasich or some other Republican.

You can quibble with those odds, obviously, and particularly with how much of a shot Trump would have against Clinton. Prudence would suggest that Trump’s chances are not zero: There could be an economic collapse, a terrorist attack or a Clinton scandal that could swing the election to Trump — or even if not, he could continue to rewrite the political rulebook and make fools out of political prognosticators. I wouldn’t take Trump at even money, though. For now, we’ll stick with the betting markets’ view, which imply that he’d be something like a 2-to-1 or 3-to-1 underdog.

The next step is figuring out what sort of person might be appointed to the Supreme Court in each circumstance. Suppose that, from Republicans’ view, a reliably liberal justice (say, someone with Justice Ruth Bader Ginsburg’s views) would score as a minus 10, and a reliable conservative (say, a clone of Justice Samuel Alito) would score as a plus 10. A truly centrist justice, who would side with the conservative position half the time and the liberal one half the time in key cases, would be a zero.

You’d probably score the possibilities something like what I have in the table below. With a Democratic Senate, Clinton would have relatively free rein to pick a nominee, although her majority in the Senate might not be especially large, and Republicans could filibuster her choice if Democrats win fewer than 60 seats. She’d also have won a fresh mandate even if Republicans held the Senate, however, and could probably get someone to Garland’s left confirmed, if not another Ginsburg.

SCENARIO PROBABILITY SCORE President Clinton + Democratic Senate 40% -9 President Clinton + Republican Senate 30 -6 President Trump! 20 +2 President Cruz or some other Republican 10 +9 Weighted average -4 The Republican calculus for a Supreme Court justice

Trump is a harder case to fathom. One way he could win the general election is by pivoting dramatically to the center and railing against partisanship. The Supreme Court nomination would then be one of President Trump’s first chances to demonstrate his abilities as a pragmatic deal-maker. He could nominate a conservative, but he could also pick someone with moderate or eccentric political views. Or he could make an unconventional choice: Trump once said his sister would make a “phenomenal” Supreme Court justice. A Trump appointment might be better for Republicans than a judge chosen totally at random from a circuit court, but perhaps not by much.

If you take a weighted average of these probabilities, you come up with a score of about negative 4. That’s equivalent to a center-left nominee – someone a lot like Garland, perhaps. If they wait until next year, Republicans might do better, but they could potentially do a lot worse.

And that’s before considering that the Supreme Court nomination isn’t happening in a vacuum. Polling suggests that a majority of the public wants the Senate to hold hearings on the next justice. Thus, blocking the appointment of Garland could hurt Republicans at the margin and further reduce their chances of keeping the Senate. On the flip side, it could curry favor with the Republican base. But one of the apparent lessons of this election is that the Republican base is neither as large nor as influential as we previously believed. Republicans continue to double down on a set of political strategies that seems to be failing.