WASHINGTON — The seed for Senator Bill Cassidy’s decision to find Donald J. Trump guilty of inciting an insurrection was planted one day last fall, when he received an email from a friend that was full of the then-president’s false claims about a stolen election.

Alarmed that Mr. Trump’s lies were gaining credence, Mr. Cassidy, a Louisiana Republican, became part of a small minority in his party — and one of only a few officials in the South — to acknowledge President Biden’s victory. Months later, after Mr. Trump’s campaign to overturn the election culminated in the Capitol riot, Mr. Cassidy was one of only seven Republican senators who voted on Saturday to convict him.

Taken at face value, Mr. Cassidy — a conservative, newly re-elected physician with a quirky streak — has little in common with the other six senators who broke with their party and found Mr. Trump guilty in the most bipartisan vote for a presidential impeachment conviction in United States history. Most were facing intense backlash on Sunday from Republicans in their states livid about the vote, as have the 10 House Republicans who supported the impeachment last month.

But the senators were united by a common thread: Each of them, for their own reasons, was unafraid of political retribution from Mr. Trump or his supporters.

“Two are retiring, and three are not up until 2026, and who knows what the world will look like five years from now,” said Whit Ayres, a veteran Republican pollster. “It looked pretty different five years ago than it did today. All seven of them have a measure of independence that those who have to run in 2022 in a closed Republican primary just don’t have.”

For Mr. Cassidy, it was a sense of outrage at the former president’s actions, starting long before the assault on Jan. 6, that played the dominant role. In an interview on Sunday, Mr. Cassidy said Mr. Trump had “trumpeted that lie” about the election for months, then sat by for hours as lawmakers and his own vice president were under attack in the Capitol and did nothing — other than to call Republican senators to ask them to continue challenging the election results.

“That anger simmers in the background,” Mr. Cassidy said. “My whole life, reading about great men and women who sacrifice for our country, who sacrifice so that we could have the freedoms that we have here today — and the idea that somebody would attempt to usurp those and destroy them?”

“It still angers me,” he continued. “It just angers the heck out of me.”

Many Republicans privately shared Mr. Cassidy’s rage, but the fact that only seven of them were ultimately willing to find Mr. Trump guilty underscored the extraordinary fealty the former president still commands in the party.

Even with Mr. Trump out of the White House, Republican lawmakers have been reluctant to cross the former president for fear of invoking his wrath and infuriating the primary voters who still adore him. All but one of the Republicans who voted to convict Mr. Trump will not face voters at the ballot box for years — or ever again, in the case of two who are set to retire in 2022.

Mr. Cassidy won re-election in November, as did two others who voted to convict the former president — Senators Susan Collins of Maine and Ben Sasse of Nebraska — meaning they have five years before their names will appear on a ballot. Two others, Senators Richard M. Burr of North Carolina and Patrick J. Toomey of Pennsylvania, are retiring. The other two, Senators Lisa Murkowski of Alaska and Mitt Romney of Utah, the 2012 Republican presidential nominee, have long since established their willingness to break with their party, and particularly with Mr. Trump.

Ms. Murkowski is the only one of the group facing re-election next year, making her vote the most politically risky of them all.

She famously returned to Washington even after losing a Republican primary in 2010 by defeating both the Republican and Democratic nominees in an audacious write-in campaign, and she has appeared untroubled by the potential political consequences of her vote.

That might be partly influenced by a change in Alaska’s voting system: Voters in November approved a measure to eliminate party primaries and institute a ranked-choice contest in which any candidate could prevail, blunting the influence of the hard-right voters who decide most Republican primaries.

At the Capitol on Saturday, Ms. Murkowski said she owed it to her constituents to vote the way she did. “If I can’t say what I believe that our president should stand for, then why should I ask Alaskans to stand with me?” she told reporters.

And in a blistering statement on Sunday, Ms. Murkowski explained why she deemed Mr. Trump guilty.

“If months of lies, organizing a rally of supporters in an effort to thwart the work of Congress, encouraging a crowd to march on the Capitol, and then taking no meaningful action to stop the violence once it began is not worthy of impeachment, conviction and disqualification,” she said, “I cannot imagine what is.”

Republicans had regarded Ms. Murkowski as a senator who was likely to defect, along with Ms. Collins. The two have previously linked arms to break from their party on significant votes, including when they helped tank a Republican-led effort to dismantle the Affordable Care Act. Ms. Collins was re-elected in November, triumphing in a brutal contest that few expected her to win, as voters reaffirmed their embrace of her long-held independent streak.

“This impeachment trial is not about any single word uttered by President Trump on Jan. 6, 2021,” Ms. Collins said in a speech from the Senate floor on Saturday. “It is instead about President Trump’s failure to obey the oath he swore on Jan. 20, 2017. His actions to interfere with the peaceful transition of power — the hallmark of our Constitution and our American democracy — were an abuse of power and constitute grounds for conviction.”

In the weeks before the impeachment trial, Ms. Collins huddled in multiple Zoom meetings with a team of lawyers, including external advisers and members of her staff, to discuss the constitutionality of putting a former president on trial and whether Mr. Trump could mount a defense premised on his right to free speech, according to Richard H. Fallon Jr., a Harvard Law professor and adviser to Ms. Collins who participated in the discussions.

“I don’t think there was any substantial disagreement at the end about the constitutional points,” he said.

Mr. Cassidy’s vote to convict was less expected. A gastroenterologist who was re-elected easily in November to a second term, he is a reliable conservative. But he has shown an increasing willingness in recent weeks to buck his party in an attempt to work with Mr. Biden and his Democratic colleagues, and markedly less interest in humoring Mr. Trump.

That approach has resulted in an intense fallout at home. The Louisiana Republican Party on Saturday moved to censure him for his vote, and Mr. Cassidy said people would be “aghast at how negative” the comments on his Facebook page had become.

But he also said that he had received “a heck of a lot of support” in texts and calls from constituents — and that he expected that sentiment to grow.

“The president spent two months building this up,” Mr. Cassidy said. “It’s going be hard; people just don’t flip on a deeply held belief from someone who they trust just like that. But the more the facts come out, the more that people will move to this position.”

For his colleagues who are retiring, voters’ reactions were less of a concern. Neither Mr. Burr nor Mr. Toomey was a particularly vocal critic of Mr. Trump while he was in office, and both skewed fiercely conservative on policy matters, especially Mr. Toomey, a fiscal hawk and former president of the pro-business Club for Growth.

But both have tangled with the former president in their own ways. As Mr. Trump continued to falsely claim that he had won the election, Mr. Toomey sharply pushed back and went so far as to blast his own colleagues for trying to overturn the results.

Mr. Burr, then the chairman of the Intelligence Committee, subpoenaed testimony from Donald Trump Jr. in 2019 as part of his work conducting the only bipartisan congressional investigation into Russian election interference. The former president’s son responded by starting a political war against the senator in an attempt to turn his party against him.

Perhaps the most predictable votes came from two of Mr. Trump’s most biting critics in the Senate: Mr. Sasse and Mr. Romney, who was the only Republican to vote to convict Mr. Trump in his first impeachment trial.

While the two senators have employed similarly scathing language to excoriate the former president, they are at very different points in their careers. Mr. Romney, 73, having tried and failed to reach the White House, has positioned himself as an elder statesman trying to steer the party from Mr. Trump’s influence regardless of the political fallout. Mr. Sasse, 48, a younger and ambitious up-and-comer, has staked his hopes on leading a post-Trump Republican Party.

Now, Mr. Sasse is facing censure threats from the Nebraska Republican Party. An effort last year by a Republican legislator in Utah to censure Mr. Romney for his first impeachment vote fell flat after the state’s Republican governor defended the senator, who faces re-election in 2024.

It is unclear how much the seven senators discussed the verdict before the vote on Saturday. But Mr. Cassidy quietly shared his decision with Mr. Burr during the closing arguments of the trial, surreptitiously passing the North Carolina Republican a note on the Senate floor.

“I am a yes,” it read.

Mr. Burr nodded in silent agreement.

Emily Cochrane and Nicholas Fandos contributed reporting.