Four votes. That’s how many Senate Republicans the Democrats have to sway if they want to pass rules for an impeachment trial they consider to be more equitable.

Approving these rules is the first order of business for a Senate trial, though Democrats and Republicans currently remain at an impasse over what they’d like to see. The ongoing fight about these procedures, which will govern things like the daily timing of the trial for President Donald Trump, whether witnesses are called, and other process questions, is set to continue when lawmakers are back from recess on January 3.

Were the 47-member Democratic caucus to stick together, they would still have to convince a handful of their GOP counterparts to buck their party in order to hit the 51 votes required to approve the Senate rules. Similarly, Republicans couldn’t afford to lose more than two members of their 53-person conference. According to a spokesman for Senate Majority Leader Mitch McConnell, any resolution that gets a 50-50 tie vote would fail.

Thus far, there are three obvious Republican contenders. Sens. Mitt Romney, Lisa Murkowski, and Susan Collins each signaled some willingness to break from the GOP when they didn’t sign a resolution condemning the House impeachment inquiry earlier this year — and they’re widely viewed as being among the only Republicans who’d be willing to do so again.

A fourth, Sen. Lamar Alexander, has also been floated by Democrats, according to Politico. And Senate Minority Leader Chuck Schumer is bullish that there are a handful of others who have private reservations about conducting a trial that’s pushed through along partisan lines. “The universe is larger than you think,” he told reporters in December.

These swing votes are expected to be pivotal, given Democrats’ and Republicans’ competing demands for the trial rules.

As of now, Democrats are interested in approving rules from the get-go that would include the testimony of at least four witnesses such as acting White House Chief of Staff Mick Mulvaney and former Trump National Security Adviser John Bolton. Republicans, meanwhile, would prefer to punt the question on witnesses until later in the trial, a move that could mean they wouldn’t have to deal with the issue at all if they’re successful at pushing through a hasty acquittal.

A vote on these rules effectively kicks off the impeachment trial and offers a glimpse of how partisan these proceedings will continue to be. During former President Bill Clinton’s trial, the initial rules on procedure were adopted unanimously, though a later motion about witness testimony only passed along party lines.

Whether any Republican lawmakers actually break with their party is an open question. In the House, not a single Republican defected to vote in favor of the articles of impeachment, and the Senate could well see a similar dynamic.

The disagreement over Senate trial rules, briefly explained

The Senate trial rules have historically been approved — at least in part — as the impeachment trial is getting underway, and they lay out process specifics.

These details include the amount of time that will be allocated for opening arguments during the trial, the procedure senators must follow to ask questions, and the protocol for calling witnesses. During Clinton’s trial, lawmakers opted to approve a resolution that tackled the procedural questions first, and only passed a motion dealing with witness testimony once the trial was underway because it was considered more contentious.

This time around, Democrats are interested in approving one resolution before the start of the trial that outlines both topics. So interested, in fact, that House Speaker Nancy Pelosi has said she won’t send the Senate the articles of impeachment the House passed at the end of last year until she sees evidence the trial will meet Democrats’ definition of fair.

“To have a trial with no witnesses and no documents is a sham trial,” Schumer said in December. Senate Majority Leader Mitch McConnell has told Fox News the GOP won’t rule out such testimony, though that’s about as far as he’s gone.

Democrats’ worries about fairness have most recently stemmed from comments by McConnell, who’s said he’s working in “total coordination” with the White House when it comes to trial protocol. McConnell has also emphasized that he’s interested in a quick trial that pretty much gets things over with.

Republicans argue that lawmakers should follow the same process that was used during Clinton’s impeachment, which involved considering a set of rules on timing first and witnesses later.

“My friend the Democratic Leader continues to demand a new and different set of rules for President Trump,” McConnell said in December.

Murkowski, Romney, and Collins are the trio of Republicans to watch

Murkowski, Romney, and Collins set themselves apart on the issue of impeachment when they refused to sign on to a resolution that condemned the procedure behind the House inquiry last year. Of the 53 members of the Republican caucus, they were the only three who declined to back it.

Additionally, they’ve been among the most vocal in expressing concerns about Trump’s conduct with Ukraine and the way that Republican leadership has talked about the impeachment trial.

Over the holidays, Murkowski said she was “disturbed” about McConnell’s comments about working with the White House on the trial process, and Collins, too, added that she was open to witness testimony. Romney has refrained from taking a more definitive stance about the trial so far, though he’s previously issued some of the strongest GOP condemnations of Trump’s behavior.

By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling. — Mitt Romney (@MittRomney) October 4, 2019

It remains to be seen if any of this strongly worded criticism becomes anything more than that. Even moderate House Republicans who found Trump’s conduct troubling — for example, Texas Rep. Will Hurd — ultimately determined that there wasn’t sufficient evidence for them to break with their party on the subject.

All three senators are also in slightly different politically positions: Neither Romney nor Murkowski faces an immediate reelection fight, while Collins is trying to hang onto her seat in Maine.

Additionally, while this trio is seen as the lawmakers most likely to flip, Democrats need at least one more Republican to swing their way if they want to guarantee a successful rules vote. Although Alexander, a more moderate Republican who’s retiring next year, is seen as a contender, he’s publicly been more circumspect in his statements on the subject.

The coming week, especially, is expected to reveal more about where lawmakers stand and set the tone for the upcoming trial itself.