WASHINGTON – Sen. Mitt Romney sat sternly with his hands clasped in his lap Wednesday as Republican colleagues to his left and right stood up and declared President Donald Trump “not guilty” on two articles of impeachment.

One of the last to enter the Senate chamber before the historic vote, Romney had, less than three hours before, announced he would be the only member of the president’s party not to acquit him on both charges. Trump, the senator concluded, had committed an act so extreme and egregious it could not be ignored.

Wearing a navy blue suit with an American flag pin, Romney had not been greeted by his colleagues and did not boast a smile as the voting began.

When the trial ended, the Utah senator was the first to depart.

Impeachment vote:"Appalling abuse": Sen. Mitt Romney votes to convict President Trump on abuse of power charge

Romney hadn’t gamed out how to navigate what would come next.

After he’d announced his decision, Romney’s chief of staff told him it was time to shift to the second part of the plan – dealing with the backlash.

“Oh? What’s that?” Romney asked.

"We don’t have one,” the aide responded, according to Romney.

Dwindling voices of dissent

Romney’s decision to become the first senator in U.S. history to vote for removal of a president of his own party now makes him a test case on whether the GOP is willing to accommodate dissent against a president who demands loyalty.

Romney joined Senate Democrats in voting to convict on an article accusing Trump of abusing his power by trying to pressure Ukraine into announcing investigations of political rivals, including former Vice President Joe Biden, in exchange for releasing nearly $400 million in military aid. Romney voted against the second article charging Trump with obstruction of Congress. Trump was acquitted on both articles.

Read Romney's full speech:Utah senator explains decision to vote to convict Trump

There aren’t a lot of alternative voices left in the GOP since Trump took office three years ago. Gone from the Senate are John McCain of Arizona, Bob Corker of Tennessee and Jeff Flake of Arizona. Trump’s most outspoken conservative critic in the House, Rep. Justin Amash of Michigan, left the GOP last year to become an independent.

Romney entered the Senate last year with the clout of having been the party’s 2012 nominee and the freedom of coming from a state that, while heavily Republican, is not in lockstep with Trump. And most in his party had accepted his criticism of Trump over the years, even if many did not agree.

Wednesday’ impeachment vote felt different. And the criticism was swift and harsh.

Broadside from Trump at prayer breakfast

The president’s oldest son, Donald Trump Jr., tweeted that Romney was "bitter" over his failed 2012 president bid and intent on impressing elites and the mainstream media. He added that Romney was officially "a member the resistance & should be expelled from the @GOP.”

On Thursday, Trump began his own onslaught against Romney.

"I don't like people who use their faith as justification for doing what they know is wrong," Trump said at the National Prayer Breakfast, a high-profile gathering of hundreds of faith leaders from around the country.

Though he did not mention Romney by name at the breakfast, there was no mistaking the target. The senator, a devout Mormon, had said in his floor speech that his "oath before God" was at the heart of his decision on impeachment.

Later, at a White House event to celebrate his acquittal, Trump was more direct, suggesting Romney was disgruntled over not having won the White House.

“Things can happen when you fail so badly,” Trump said. “I’m sorry about Mitt Romney.”

Matt Schlapp, head of the American Conservative Union and a staunch Trump ally, said Utah now has a “big problem” that his group would like to help fix.

“#DumpRomney,” tweeted Schlapp who had already banned Romney from attending this year’s gathering of conservative activists after Romney was one of only two Republican senators to vote for witnesses at the Senate impeachment trial.

'Another day and another vote'

Romney's Senate colleagues were generally more muted in their response, saying it would not be productive to ostracize him.

"We don't have any doghouses here," Senate Majority Leader Mitch McConnell, R-Ky. He said Romney had been "largely supportive of most everything" on the GOP's congressional agenda.

But McConnell said he wished Romney hadn't voted the way he did.

“I was surprised and disappointed,” the Senate GOP leader said.

Sen. Susan Collins, R-Maine, a key swing vote who voted to acquit the president, said while she disagreed with Romney, she has "a lot of respect" for him and did not think his decision would make his role in the Republican conference uncomfortable.

"I respect his right to vote his conscience, and I don't think there ought to be retribution," agreed Sen. John Cornyn, R-Texas, the second most powerful Senate Republican.

Lone ranger:Mitt Romney: a solitary GOP voice battling Trump for the soul of the Republican Party

His GOP colleagues also knew Romney's history as someone who had openly sparred with Trump on his way to the presidency. In 2016, Romney held a news conference to denounce the future president as a "phony" and a "fraud."

"Well, I think he and the president had a little bit of a complicated relationship to start with,” GOP Sen. John Thune of South Dakota said with a smile. “In the (GOP) conference, he’s somebody that we all know is a very independent person. And obviously, you know, we’re going to continue to work with him. There’s always going to be another day and another vote."

Wrestling with arguments, losing sleep

A feeling of “real dread” had swept over Romney when House Speaker Nancy Pelosi last fall launched Democrats’ impeachment inquiry.

Romney realized he was going to have to sit in judgment on the leader of his party, someone whose policies he backs the vast majority of the time, and – despite a complicated history with Trump – someone for whom he had “feelings of cordiality.”

“We are collegial,” Romney said of Trump, even though the freshman senator had published an opinion piece when he took office last year warning that Trump “has not risen to the mantle of the office.”

“That’s where he basically said, ‘I want to be kind of the moral voice,’” said Lara Brown, director of George Washington University’s Graduate School of Political Management. Romney was saying, “I agree with Trump on a lot of issues. But this isn't always about issues. This is about principles,” Brown said.

But there’s a canyon of difference between penning an op-ed and voting to find the president guilty of an impeachable offense.

When Trump’s trial began in the Senate, Romney never slept past 4 a.m. His mind churned through the arguments presented by both sides and the consequence of the evidence he was given.

'15 minutes of fame'

At a closed-door Republican lunch during the trial, Romney stood up to address his colleagues. A hush fell over the ornate dining room. Some senators stopped eating. They peered up at the former presidential candidate.

Romney argued that it was important to hear from eye witnesses, such as former national security adviser John Bolton, who could dispute or confirm the accusations against Trump.

After the lunch, Romney was denounced on Twitter by the newest member of the Senate, Kelly Loeffler, R-Ga.

“Sadly, my colleague @SenatorRomney wants to appease the left by calling witnesses who will slander the @realDonaldTrump during their 15 minutes of fame,” Loeffler tweeted.

Loeffler had backed Romney’s 2012 presidential bid, including contributing with her husband a combined $1.5 million to a pro-Romney super PAC, according to the nonpartisan Center for Responsive Politics.

“I can’t tell you the number of people who’ve stopped me and said, ‘I gave my largest donation when Mitt Romney was running for president and I really regret that. I don’t understand what he’s doing,’” said David McIntosh, head of the Club for Growth, a conservative group that had opposed Trump’s 2016 bid but now backs his reelection.

During the impeachment process, the group aired ads in Utah to warn Romney and “probably half a dozen colleagues in the Senate” they were being closely watched, McIntosh said.

Bill Kristol, a leader of the conservative resistance to Trump, had offered “a little bit of backup” for Romney in ads run in Utah and some other states urging Republicans to “let Bolton testify.”

“It doesn’t hurt for there to be a little bit of messaging on his side,” Kristol said. “For me, the story has always been the Republicans on the Hill and the pretty amazing degree to which they have decided to just capitulate to Trump.”

Dear fellow Republicans

In letters he gave to each of his GOP colleague before his floor speech, Romney wrote of the role his faith had played in his thinking on impeachment.

“I am a profoundly religious person,” he wrote at the top of the letters, which were signed with blue ink and delivered to senators' boxes in the cloakroom.

During his speech, the Utah senator choked up for a few seconds as he explained how his Christian background affected his vote.

"The allegations made in the articles of impeachment are very serious," he said. "As a Senator-juror, I swore an oath, before God, to exercise 'impartial justice.' I am a profoundly religious person. I take an oath before God as enormously consequential. I knew from the outset that being tasked with judging the president, the leader of my own party, would be the most difficult decision I have ever faced. I was not wrong."

He'd reached his conclusion after senators had a chance to ask questions of both sides last week and started drafting what he would say to explain a vote that "will have impact on me and my family for a long, long time to come."

“I am sure to hear abuse from the president and his supporters,” Romney said in his floor remarks. “Does anyone seriously believe I would consent to these consequences other than from an inescapable conviction that my oath before God demanded it of me?”

'We all need each other'

After the votes were cast and counted over and the president had been acquitted, Romney quietly rose from his desk and left chamber into an uncertain political future.

He went downstairs to the line of cars awaiting the senators to come out, hopped into his and motored away from the Capitol.

"It'd be hard to blame him if he wanted to sort of avoid a lot of us for a while,” said Sen. Kevin Cramer, who sits in the back of the Senate chamber near Romney. “It might have been awkward, but I would have tried to do my best to not make it that way. We all need each other too much in the Senate.”

Contributing: Nicholas Wu, Savannah Behrmann