Inside the Mansfield Room at the Capitol, where Senate Republicans have held daily strategy sessions during President Trump’s impeachment trial, senators began walking through their arguments on why they should block witnesses from testifying in the proceedings.

But at this lunch on Thursday, Sen. Mitt Romney (Utah) was getting visibly annoyed.

If this is meant to persuade me, Romney told his colleagues, it’s not helpful, according to two officials with knowledge of the exchange. The senator, a near-lone GOP voice in seeking witnesses for the trial, felt as if other Republicans were singling him out.

That private remark illustrated how Romney has become a rarity in the Senate GOP conference and his party — the man Republicans rallied around for the presidency in 2012 was an outlier in a GOP bound to Trump and unwilling to challenge the president. That political reality helped Senate Majority Leader Mitch McConnell (R-Ky.) hold his conference together in preventing witnesses.

Ultimately, Romney and Sen. Susan Collins (R-Maine) — who faces a tough reelection in a Democratic-leaning state — sided with Democrats, but other key Republicans such as Sens. Lisa Murkowski (Alaska) and Lamar Alexander (Tenn.) aligned with the rest of the conference on the critical vote Friday, paving the way for Trump’s likely acquittal this week on charges of abuse of power and obstructing Congress.

“There’s never been arm twisting,” Sen. Mike Braun (R-Ind.) said. “It was going to disproportionately rest on a few shoulders because most in each conference were pretty clear what they wanted to do.”

This account of how McConnell and enough Senate Republicans blocked witnesses from entering Trump’s trial — the issue that was most in question during the contentious proceedings — is based on interviews with 13 senators and other officials familiar with the deliberations who spoke on the condition of anonymity to talk frankly.

Trump’s acquittal was never in question in the Republican-controlled Senate, but the uncertainty about whether to call witnesses — as had been done in each of the previous 15 impeachment trials — created last-minute drama amid new revelations about Trump’s move to strong-arm Ukraine into investigating his domestic political rivals.

In the end, McConnell held his conference together, arguing that witnesses would drag the trial out for weeks and delay other Senate work. Several Republicans acknowledged that the president did use nearly $400 million in military aid to Ukraine as leverage to benefit himself politically, calling it inappropriate, but argued it wasn’t grounds to oust him from office.

“What was, I think, the most persuasive was just the open-ended consequences of starting down that path, and particularly the delays inherent in litigation that would ensue in the middle of the trial,” said Sen. John Cornyn (R-Tex.).

McConnell was among the first to argue that Republicans should avoid calling witnesses despite Trump’s clamor for the whistleblower whose report triggered the House impeachment probe, former vice president Joe Biden and his son, Hunter, to testify. The leader warned in a mid-December lunch that a protracted witness fight would be dangerous for both parties.

“Mutually assured destruction,” he told them.

But McConnell, navigating a narrow 53-47 majority, was contending with the influential clutch of four senators who — had they and others banded with Democrats to call witnesses — could have stretched the trial proceedings for weeks with fraught political ramifications for the Republican Party. Early on, the four prevailed in getting McConnell to include a vote on witnesses in the measure setting the scope of the proceedings.

Just days before the trial began, former national security adviser John Bolton stunned Washington when he said he would be willing to testify before the Senate if subpoenaed. The ex-Trump official, who had likened the Ukraine pressure scheme to a “drug deal,” according to one witness, left a message with McConnell to alert him of his announcement, but the senator never returned the call.

The news, however, got Romney’s attention. Receiving testimony from Bolton was a “no-brainer,” he believed, according to an individual familiar with his thinking who, like others interviewed for this story, requested anonymity to speak frankly.

 “Here we have somebody who has obviously talked about this issue with the president who may have helpful information,” the person said, summarizing Romney’s thinking. “So why wouldn’t we want to hear from him?”

But there were other currents working against Romney. An idea promoted by Sen. Ted Cruz (R-Tex.) of a Republican witness for every one proposed by Democrats was gaining traction within the conference — a clear signal from Republicans that any successful attempt in the Senate to summon witnesses would devolve into a messy, prolonged fight.

Cruz and other Republicans met privately with McConnell in mid-January to pitch him on the idea. The majority leader was on board quickly, Cruz recalled.

“He and I have famously disagreed in other contexts,” said Cruz, who called McConnell a liar in a 2015 showdown on the Senate floor. “But in this instance, both of us were working to keep the conference together and ensure the outcome of the trial was the outcome dictated by the Constitution, which was an acquittal because the House managers hadn’t proven their case.”

Meanwhile, McConnell was working to ensure Trump and the White House trusted him to handle the trial strategy as he dealt with a mercurial president who had his own ideas about the proceedings. In one phone call shortly before Christmas, McConnell bluntly told Trump that while the president was getting a lot of feedback about how the trial should be conducted, he knew the Senate better than anybody who had been advising the president and, most importantly, how to make his members comfortable.

McConnell told Trump that he needed to trust him, according to a person familiar with the conversations who spoke on the condition of anonymity to talk frankly. Trump responded that he did.

The administration mostly left the wavering senators alone — namely out of McConnell’s insistence. He warned Trump in the fall not to alienate moderates lest he make the situation worse, according to Republicans.

“The White House has not asked for calls,” said one senior GOP official close to the moderate senators. “They’ve not asked for meetings. They’ve not texted.”

Indeed, the last time Alexander and Trump spoke directly was on Dec. 19 at a private Oval Office education bill signing. Trump summoned an aide to bring him a copy of the six-page letter he had fired off to House Speaker Nancy Pelosi (D-Calif.) two days prior in which he repeatedly criticized the impeachment process. The president wanted Alexander to read the letter, complaining that what the Democrats were doing to him was ridiculous, but the senator deflected and shifted to the topic at hand: the bill being signed into law.

Early in the process, the four senators — Romney, Collins, Alexander and Murkowski — often talked strategy while McConnell found various ways to cater to them. For example, he gave Collins, Murkowski and Romney the first shot at querying the legal teams during the trial’s question-and-answer session.

Once the trial started, the swing votes stayed in touch during breaks in the proceedings, but otherwise, they primarily kept their own counsel.

Alexander wouldn’t give any hints of how he would vote on the pivotal witness question to McConnell — his closest friend in the Senate — until he privately informed the majority leader during a dinner break on Thursday evening. Aware that Collins and Romney would likely vote in favor of witnesses, Alexander also sat down with Murkowski later that night, informing her of his decision.

Unlike other issues in the Senate, there were no substantive attempts to broker a bipartisan agreement that would cool tensions, ensuring that the impeachment proceedings began — and ended — as a near-partisan exercise.

“It has just been a lot harder over the last two weeks to have conversations with Republican friends,” said Sen. Chris Murphy (D-Conn.). “The mood in that room is super tense and super heavy.”

Democrats heavily pressured vulnerable Senate Republicans up for reelection this fall, driving a message declaring any trial without witnesses would be a “coverup.” But GOP senators and aides were confident that voters were paying little attention based on a survey conducted by the National Republican Senatorial Committee, the official campaign arm of Senate Republicans.

“You know, Iowans right now — honest to God — they want us to get this thing over with,” said Sen. Joni Ernst (R-Iowa). “They’re like, ‘Why aren’t you doing the business of the American public?’ ”

In a closed-door party meeting in McConnell’s suite of offices — where GOP senators have held some of their most consequential strategy sessions — it was those precise swing-state Republicans like Sens. Cory Gardner (R-Colo.) and Thom Tillis (R-N.C.) who said they were prepared to just move on to a final vote without calling additional witnesses.

That was after Bolton’s blockbuster allegation, laid out in an unpublished manuscript of his book as reported by the New York Times, that Trump directly tied the holdup of aid to Ukraine to the political investigations.

The Times report rocked the GOP conference and made Romney believe it increased the likelihood that more Republicans would join in his effort to call Bolton. At a Jan. 27 lunch, Sens. Patrick J. Toomey (R-Pa.) and Bill Cassidy (R-La.) spoke up about a witness trade, appearing to briefly consider the notion. Romney joined them, making a forceful argument that they had a duty as senators to hear all the relevant evidence.

At that moment, the GOP conference was in chaos reacting to the Bolton news, with leaders worried for the first time that they might actually lose the witness vote. During the meeting, McConnell warned members to stay calm and hear out Trump’s defense team.

The majority leader delivered an even sharper message in a private meeting the following day. While McConnell, who faces his own reelection this year, often frames his argument in the context of retaining the Senate majority, he delivered a different message: The Senate exists to stop partisan fevers from jeopardizing our institutions, and Republicans must ensure that did not happen in the fight over Trump’s impeachment.

The news from Bolton’s book emerged in the trial’s question-and-answer session, as Cruz and Sen. Lindsey O. Graham (R-S.C.) worked with Murkowski and Alexander in the GOP cloakroom to devise a question to the White House counsel that Cruz and Graham hoped would help persuade the swing votes.

That question: Even if Bolton’s allegations of a quid pro quo were true, isn’t that still not an impeachable offense, so would his testimony add anything? Patrick Philbin, a deputy White House counsel, responded that it would not be an impeachable offense.

“That answer was pivotal, especially in getting Lisa’s vote,” Cruz said.