WASHINGTON — In the end, the impeachment calculation nearly all Senate Republicans are making is fairly simple: They would rather look like they ignored relevant evidence than plunge the Senate into an unpredictable, open-ended inquiry that would anger President Trump and court political peril.
As Republicans on Wednesday lined up behind blocking witnesses in the trial, their reasoning reflected the worry that allowing testimony by John R. Bolton, the former national security adviser whose unpublished manuscript contradicts a central part of Mr. Trump’s impeachment defense, would undoubtedly lead to a cascade of other witnesses. They in turn could provide more damaging disclosures and tie up the Senate indefinitely, when the ultimate verdict — an all but certain acquittal of the president — is not in doubt.
“For the sake of argument, one could assume everything attributable to John Bolton is accurate, and still the House would fall well below the standards to remove a president from office,” said Senator Lindsey Graham, Republican of South Carolina.
Republicans have offered myriad rationales for refusing new testimony: Gathering it was the House’s job. Calling more witnesses would lead to prolonged court fights over executive privilege. They had heard more than enough evidence to reach a verdict. There was not enough evidence to show they needed more information. Allowing the House to force the Senate into a drawn-out impeachment trial would set a dangerous institutional precedent. In essence, during what they hoped would be the final hours of Mr. Trump’s trial, Senate Republicans were constructing a permission structure for not trying to get to the bottom of what happened, with the hope that voters would find their explanations satisfactory and reasonable.
“We don’t need Mr. Bolton to come in and to extend this show longer, along with any other witnesses people might want, and occupy all of our time here in the Senate for the next few weeks, maybe even months,” Senator John Cornyn, Republican of Texas and a close ally of Senator Mitch McConnell of Kentucky, the majority leader, said Tuesday evening on Fox.
Josh Holmes, a former chief of staff and a top outside adviser to Mr. McConnell, made it clear that Republicans viewed the idea of calling witnesses as a disaster in the making.
“More witnesses = Hindenburg,” Mr. Holmes wrote Wednesday on Twitter, showing a picture of the flaming airship. “None of it changes ultimate acquittal.”
Mr. McConnell has maneuvered to head off the conflagration. In a private meeting with senators on Tuesday, he warned rank-and-file Republicans that he was short of the votes to thwart a Democratic call for witnesses, an unmistakable tactic to bring wavering senators into line.
On Wednesday morning, he summoned a key swing vote, Senator Lisa Murkowski of Alaska, to his office for a private meeting. She emerged refusing to speak about her intentions. And when the question-and-answer period opened later in the day, he gave the first question to three of the remaining Republican holdouts for witnesses: Senator Susan Collins of Maine, Ms. Murkowski and Senator Mitt Romney of Utah. The move signaled that Mr. McConnell was singularly focused on providing the waverers the answers they needed to feel comfortable ending the trial without more evidence.
Nearly all of the politically vulnerable Senate Republicans up for re-election in November have embraced their party’s strategy, making it clear that they favor taking their chances defending their votes against witnesses over trying to explain to voters loyal to Mr. Trump why they backed broadening an investigation into a president who is very popular with the Republican electorate.
Senator Cory Gardner, the Colorado Republican trying to hang on to his Senate seat in a state that has turned increasingly blue, illustrated that point on Wednesday when he said he would not support additional witnesses — a stance that is likely to draw considerable blowback from critics at home but endear him to Republicans.
“I do not believe we need to hear from an 18th witness,” he said in a statement, emphasizing that the House had already heard from plenty of people.
While polls show broad bipartisan support for calling new witnesses, Mr. Gardner, who protectively endorsed Mr. Trump’s re-election months ago, is keenly aware that he stands no chance without the wholehearted backing of the president. Republicans live with the reality that a critical tweet from the president can quickly send their campaigns into a tailspin, a point reinforced by the president’s latest Twitter warning shot on Wednesday morning.
“Remember Republicans,” he wrote, “Witnesses are up to the House, not up to the Senate. Don’t let the Dems play you!”
Senator Collins is the only Republican up for re-election who is now seen as a likely vote for more witnesses. She is the rare member of her party who still seeks to appeal to a broad range of independent and even Democratic voters as well as Republicans. Her fellow Republicans say they see her as being in a unique position, and they have given her ample running room to do what she thinks is best for her re-election, even if it causes them problems.
Republicans insist that they have given the House case against Mr. Trump serious and sober review and have found it wanting. They are also obviously chafing against the constraints of the trial, forced to sit quietly in the chamber for hours on end, when they are much more accustomed to making their presence known at hearings and during floor votes and then exiting at their convenience. The thought of the trial continuing, with no end in sight and the result preordained, sparks despair in many of them.
But it is not just their schedules they see at risk if the Senate were to go down the path of new witnesses. Republicans have increasingly pointed to the fact that the Democratically controlled House has forced the Republican-led Senate into an impeachment trial to the exclusion of almost all other activity under strict Senate rules. They say allowing the House to effectively freeze the Senate would set a dangerous precedent.
“To make something out of the two impeachment articles would send an incredibly bad message to every House after this,” said Senator Roy Blunt of Missouri, a member of the Republican leadership. “If you really want to shut the Senate down, just send them a vague article of impeachment.”
Democrats dismiss that complaint as well as the others raised by Republicans, saying they are simply in search of justification for failing to conduct a thorough review of the behavior of a president with a firm hold on voters who are essential to their individual political survival, as well as Republican control of the Senate.
“They keep coming up with excuses,” said Senator Chuck Schumer of New York, the Democratic leader, who added that Republicans’ claims that witnesses would chew up weeks or months of Senate time were exaggerated and that he believed the testimony could be secured and wrapped up in a week or two.
If Republicans were so worried about precedent, he said, they should be concerned with what will happen if Mr. Trump is acquitted without the Senate taking the necessary steps to parse all the information it can about his conduct.
“If he’s allowed to completely stonewall, to do absolute obstruction on everything and not be held accountable, he’ll do it again and again, and future presidents will do it again and again,” Mr. Schumer said. “And this grand experiment we call democracy will have been fatally, fatally eroded.”
Still, Mr. Schumer conceded on Wednesday that his hopes of additional witnesses were growing fainter as the Republican leadership worked to lock down senators and bring the momentous proceeding to a close as soon as the vote on whether to call witnesses was concluded — a move now expected on Friday.
A slim possibility still existed that other Republicans would join Democrats and Senators Collins and Romney in calling for more testimony, upending the party’s game plan. But nearly all Republicans were more than ready to vote, and they did not need new witnesses to confirm their verdict that it was past time to bring a speedy end to the trial.