Sen. Susan Collins of Maine, who announced last week she would run for reelection, has been more circumspect than many of her colleagues, even amid pressure from Democrats over the machinations of a trial. She disagreed with McConnell’s comments — but has also not made declarations about whether witnesses should testify, saying she hopes McConnell and Minority Leader Chuck Schumer can come to an agreement, something they failed to do before departing Washington for the holidays.

Polling has consistently shown a sharp split over impeachment, with Democrats overwhelmingly in favor and Republican voters vehemently opposed to impeaching and removing Trump from office. In a recent POLITICO/Morning Consult poll, 47 percent of independent voters supported removing Trump from office, while 40 percent opposed Trump's removal.

GOP strategists see little political upside for any Republican senator to consider breaking with the president on the issue given that they would inevitably see a massive drop-off in support among their base voters. Scott Jennings, a political adviser to McConnell and former adviser to President George W. Bush, said Republicans have little to gain by breaking with their party during a Senate trial.

"But you have a hell of a lot to lose," Jennings said. "If you look at Republicans in any state — a big Trump state or not — and you're a Republican office holder, you have a heck of a lot to lose by breaking with the president."

Jennings pointed to the lack of House Republicans supporting impeachment, including retiring members who have been critical of Trump in the past, as evidence of the political case.

"If you can’t get Will Hurd to see it your way, how are you going to get somebody in the Senate to see it your way?” he said, referring to the retiring Republican congressman from Texas.

Democratic senators and challengers have been more cautious in their approach. The party can’t win back control of the Senate without winning in several states Trump carried in 2016.

And while most of the party’s vulnerable incumbents and top challengers have criticized the president’s conduct and supported the House’s impeachment inquiry, they have stopped short of calling for his removal from office before the trial.