And the early maneuvering in the Senate trial is just as sharply partisan. Senate Majority Leader Mitch McConnell (R-Ky.) has persuaded all 53 Republicans to support his proposal to open the proceedings using the same rules that guided the 1999 impeachment trial of President Bill Clinton. Minority Leader Chuck Schumer (D-N.Y.) has repeatedly attacked McConnell’s plan, accusing Republicans of being involved in a “cover up” if they don’t allow witnesses and new documents to be used as evidence in the case.

For Trump, the issue will be whether a majority of the Senate votes for conviction or acquittal on either article of impeachment, especially as his reelection looms less than 10 months away.

Yet the key for McConnell and Schumer throughout the trial will be persuading the handful of senators in the middle to vote with them on how the trial actually plays out.

“Different people are in different political situations and different parts of the country come to it with different predispositions,” said Senate Majority Whip John Thune (R-S.D.). “You have to try and balance all of that. It’s all about the majority. It’s all about 51. It’s about how we get 51?”

Here’s a look at how the different Senate factions are shaping up:

The Three Amigos, Part Deux

The Republican triumvirate of Sens. Susan Collins (Maine.), Lisa Murkowski (Alaska) and Mitt Romney (Utah) has been the center of a media frenzy for weeks. This unpredictable trio wants the option to hear from additional witnesses on the Ukraine scandal that led to Trump’s impeachment, especially former national security adviser John Bolton. Bolton has said he’ll testify if subpoenaed, but Trump has suggested he may invoke executive privilege to prevent him or other administration officials from testifying. Trump's GOP allies have also threatened to call Hunter Biden if Bolton testifies. The heart of the impeachment case centers on Trump pressuring Ukraine to investigate his political rivals, including Joe Biden and his son Hunter.

The key issue for this moderate group — and maybe the biggest question for the entire Trump trial — is whether there will be a fourth Republican who joins them. The likelihood of that happening is pretty low at this point, however.

Collins issued a lengthy statement following what she said was “a lot of mischaracterization and misunderstanding about my position on the process the Senate should follow for the impeachment trial.”

Collins, who is up for reelection this fall, added that “I tend to believe having additional information would be helpful. It is likely that I would support a motion to call witnesses at that point in the trial just as I did in 1999.”

Murkowski, for her part, said she’s considering printing a sign to let reporters know her position — she’s not deciding about witnesses anytime soon.

“Until that point in time I’m not thinking about each individual witness,” Murkowski said last week. “Listen to the cases that are being presented, the questions that are going to be asked by members and then we’ll have a chance to say: Do we need to hear more?”

Romney — who has made clear that he’d like Bolton to testify — has vowed “to be as impartial as the oath requires.”

Republican Institutionalists

If Democrats are to get a fourth GOP vote for witnesses, their best bet is from a group of veteran Republicans who believe in protecting the Senate prerogatives or are retiring and thus have less to fear from a vengeful Trump. This group include Sens. Lamar Alexander (Tenn.), Rob Portman (Ohio), Roy Blunt (Mo.) and Pat Toomey (Pa.).

But it won’t be an easy vote for any Republican, to say the least. And there's not a lot of private discussions between both sides on this issue as well, according to senators in both parties. Either way, this bloc of Republicans could prove pivotal for the Trump proceedings.

“I won’t make a decision about that until, No. 1, we’ve heard the arguments, and No. 2, asked our questions,” Alexander said in an interview. “Then I’ll decide if I think we need more evidence about witnesses or documents” to reach a verdict.

“I think we have to have the trial, but I think it is premature now to decide whether we have witnesses. The Clinton model is a sensible model,” Toomey added. “At that point, we’ll have a whole body of information and we can decide” on witnesses.

Trump Hard-liners

Then-candidate Trump infamously joked during the 2016 campaign that “I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose voters.” While GOP senators might say they wouldn't go that far, there are a number — such as Lindsey Graham of South Carolina, Marsha Blackburn of Tennessee, Tom Cotton of Arkansas, Kevin Cramer of North Dakota, Ted Cruz of Texas and Josh Hawley of Missouri, for instance — who have blasted Speaker Nancy Pelosi, House Democrats and the media for a “sham impeachment” that isn’t going to result in Trump’s ouster. Even McConnell has said for months that “I think we all know where this going to end up.”

It’s clear right now that for well over 40 Senate Republicans, the Trump trial will be a burden to be endured for now and then exploited for political gain.

“I know people in the Republican base. I consider myself to be part of the Republican base,” said Sen. Rand Paul of Kentucky, who has lobbied McConnell to allow Trump’s lawyers to call Hunter Biden if Democrats get to call Bolton. “I think that they will be very, very, very unhappy with a Republican that allows this to go and become the circus that the House was.”

Democratic Moderates

There's a small group of Senate Democrats who are in play for GOP leaders and the White House. This group includes Sens. Joe Manchin of West Virginia and Kyrsten Sinema of Arizona. Doug Jones of Alabama, the most endangered Senate Democrat this fall, wants to hear from witnesses, but he has also called for Trump to “get a fair trial.”

Sinema has already announced that she doesn’t want to talk about the case while the trial is playing out. “I will treat this process with the gravity and impartiality that our oaths demand and will not comment on the proceedings or facts until the trial concludes,” Sinema said in a statement.