As the impeachment trial careens into the senators’ questions, the big question on everyone’s mind is: Will the Senate vote to allow witnesses — in particular, former national security adviser John Bolton? Bolton’s forthcoming book gives a firsthand account of President Trump saying the administration would withhold military aid to Ukraine until its government announced an investigation into Trump’s political rival Joe Biden — the quid pro quo central to the House’s first impeachment charge. If witnesses are called, that would kneecap GOP leader Mitch McConnell‘s plans to acquit the president by the week’s end.

Tuesday night, McConnell told GOP colleagues that he did not yet have the votes needed to block witnesses.

Here’s what you need to know about the Republicans’ predicament.

1. The best-laid plans of mice and men

McConnell’s strategy remains twofold: Keep his GOP conference united and minimize daylight between Senate Republicans and the president. At first, that meant Republicans agreed to defer votes on allowing witnesses until after both the House impeachment managers and the president’s defense rested. By then, McConnell reasoned, the public would consider the trial fair, making it easier for GOP senators in swing states to vote against witnesses.

To further insulate purple-state colleagues, Republicans adopted trial procedures that make it hard for voters to know precisely what their senators are voting on. The supplemental trial rules include a vote (expected Friday) on whether to even allow subsequent votes to subpoena particular witnesses. If Republicans stick together and vote down the first question, the Senate would never even vote on whether to subpoena specific witnesses such as Bolton.

But if the demand for Bolton’s testimony fractures GOP unity, that path gets rockier. Allowing the House managers to question Bolton could seriously slow down the trial, increase public pressure for more witnesses, and expose Republicans to Trump’s Twitter taunts and voter anger — both unwelcome in an election year.

2. Apparently it’s not just swing-state senators who are on the fence

Reports suggest that more than the usual suspects’ votes could be in play. Beyond Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska), Sens. Mitt Romney (Utah), Lamar Alexander (Tenn.) and Rob Portman (Ohio) have also reportedly signaled that they may support calling Bolton to testify.

These aren’t moderate senators by standard political science accounts. Nor do they all represent swing states. That’s important, because it suggests there could be still more senators on the fence about whether the Senate should call Bolton or other witnesses.

What’s more, the greater the number of defectors, the safer those defectors would be. No GOP senator could be singled out as the culprit for inviting Bolton’s testimony — perhaps helping to insulate Republican senators from the president’s wrath.

3. Don’t count out McConnell

McConnell may yet nail down enough GOP votes to block witnesses. The challenge for Democrats is that even if some Republicans want to hear from Bolton, the interests of Democrats and the GOP defectors are not neatly aligned. Some GOP senators will want political cover for siding with the Democrats, most likely by demanding that Joe Biden or his son Hunter be deposed alongside Bolton.

Would that be too much for Democrats? Possibly. Democrats have already said they will not support calling either Biden to testify. Republican leaders could be counting on Democrats’ resistance to calling the Bidens as the thing that would derail any agreement to subpoena Bolton. If that happened, Republicans could claim they supported calling Bolton and blame Democrats for killing the deal. How many Republicans would insist on witness “reciprocity,” however, remains unknown.

The risks are high for McConnell, who has staked his leadership and party majority on hanging together and minimizing discord with the president. Partisan interests might yet be enough to pressure Republicans to stay in the fold and defeat the call for witnesses. But the Bolton news may have shifted what the public views as a “fair trial.” If so, rising public support for calling witnesses could upend McConnell’s partisan calculus.

Read more TMC analysis of the impeachment inquiry: