WASHINGTON — A Senate still bruised from the most violent attack on the Capitol in two centuries acquitted former President Donald J. Trump on Saturday in his second impeachment trial, as all but a few Republicans locked arms to reject a case that he incited the Jan. 6 rampage in a last-ditch attempt to cling to power.

Under the watch of National Guard troops still patrolling the historic building, a bipartisan majority voted to find Mr. Trump guilty of the House’s single charge of “incitement of insurrection.” They included seven Republicans, more members of a president’s party than have ever returned an adverse verdict in an impeachment trial.

But with most of Mr. Trump’s party coalescing around him, the 57-to-43 tally fell 10 votes short of the two-thirds majority needed to convict, and allow the Senate to move to disqualify him from holding future office.

Among the Republicans breaking ranks to find guilty the man who led their party for four tumultuous years, demanding absolute loyalty, were Senators Richard Burr of North Carolina, Bill Cassidy of Louisiana, Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, Ben Sasse of Nebraska and Patrick J. Toomey of Pennsylvania.

The verdict brought an abrupt end to the fourth presidential impeachment trial in American history, and the only one in which the accused had left office before being tried. But it was unlikely to be the final word for Mr. Trump, his badly divided party or the sprawling criminal and congressional investigations into the assault.

It left behind festering wounds in Washington and around the country after a 39-day stretch unlike any in the nation’s history — encompassing a deadly riot at the Capitol, an impeachment of one president, the inauguration of another and a brief but rancorous trial in the Senate.

It took only five days to reach a verdict, partly because Democrats and Republicans were united in their desire to avoid a prolonged proceeding and partly because Mr. Trump’s allies made clear before it even began that they were not prepared to hold him responsible. Most of the jury of senators had themselves witnessed the events that gave rise to the charge, having fled for their own lives, along with the vice president, as the mob closed in last month while they met to formalize President Biden’s victory.

Party leaders and even the president’s most loyal supporters in the Senate did not defend his actions — a monthslong campaign, seeded with election lies, to overturn his decisive loss to Mr. Biden that culminated when Mr. Trump told thousands of his supporters to “fight like hell” and they did. Instead, in the face of a meticulous case brought by nine House prosecutors, they found safe harbor in technical arguments that the trial itself was not valid because Mr. Trump was no longer in office.

But their overriding political calculation was clear. After party leaders briefly entertained using the process to purge Mr. Trump from their ranks, Republicans doubled down on a bet made five years ago: that it was better not to stoke another open confrontation with a man millions of their voters still singularly embrace.

Senator Mitch McConnell, Republican of Kentucky and the minority leader, embodied the tortured balancing act, denouncing Mr. Trump on Saturday, minutes after voting to acquit him, for a “disgraceful dereliction of duty.” In blistering remarks from the Senate floor, Mr. McConnell, who had openly considered voting to convict Mr. Trump, effectively argued that he was guilty as charged, while arguing that there was nothing the Senate could do about it.

“There is no question — none — that President Trump is practically and morally responsible for provoking the events of the day,” Mr. McConnell said. “The leader of the free world cannot spend weeks thundering that shadowy forces are stealing our country and then feign surprise when people believe him and do reckless things.”

But Mr. McConnell, who refused to call the Senate back into session to hold the trial while Mr. Trump was still in office, argued that he could not be convicted once he no longer was. Mr. McConnell said the only way to punish him now was through the criminal justice system. Mr. Trump, he said, “didn’t get away with anything yet.”

Minutes after the verdict, Mr. Trump, barred from Twitter, broke an uncharacteristic silence he had maintained during the trial with a defiant statement issued from his post-presidential home in Florida, calling the proceeding “yet another phase of the greatest witch hunt in the history of our country.”

He expressed no remorse for his actions, and strongly suggested that he planned to continue to be a force in politics for a long time to come.

“In the months ahead, I have much to share with you, and I look forward to continuing our incredible journey together to achieve American greatness for all of our people,” Mr. Trump said.

The “not guilty” verdict left him free to run for office again, but it remained unclear whether he could recover after he became the first president to seriously threaten the peaceful transfer of power. Public polling suggests Republicans have pulled their support in droves since the events of last month, but an acquittal is likely to empower Mr. Trump with the party’s activist base and further stoke the party’s gaping divisions.

Democrats, for their part, condemned the verdict but intended to quickly turn Washington’s focus to the new president’s ambitious legislative agenda and the coronavirus pandemic passing grim new milestones each day. The outcome promised to leave Mr. Biden, who took office pledging to “end this uncivil war,” with the monumental task of moving the nation past one of its most violent and turbulent chapters since the 19th century.

But that did not mean party leaders were willing to forgo a potential political advantage. Speaker Nancy Pelosi quickly batted down the idea of a bipartisan censure resolution, saying it would let “cowardly senators” off the hook and constitute “a slap in the face of the Constitution.”

“Five years ago, Republican senators lamented what might become of their party if Donald Trump became their presidential nominee and standard-bearer,” Senator Chuck Schumer, Democrat of New York and the minority leader, said moments after the vote. “Just look at what has happened. Look at what Republicans have been forced to defend. Look at what Republicans have chosen to forgive.”

In a Capitol still ringed by fencing and barbed wire, the presiding officer, Senator Patrick J. Leahy of Vermont, set the question before senators shortly before 4 p.m.:“Senators, how say you? Is the respondent, Donald John Trump, guilty or not guilty?”

Seated at mahogany desks defiled just weeks before by insurrectionists in search of material they could use to stop Mr. Biden’s victory, senators wearing masks to guard against spreading the coronavirus rose in alphabetical order to cast their votes.

“It is, therefore, ordered and adjudged that the said Donald John Trump be, and he is, hereby acquitted of the charge in said article,” Mr. Leahy declared.

The vote came hours after the trial briefly dissolved into chaos when House prosecutors made, then dropped, a surprise demand for witnesses who could reveal what the former president was doing as the assault unfolded. Instead, the two legal teams agreed to admit as evidence a written statement by a Republican congresswoman who has said she was told that the former president sided with the mob as rioters were attacking the Capitol.

With the outcome a foregone conclusion, the trial itself became an illuminating and cathartic act for history, clarifying the scope of the violence that occurred.

It could scarcely have been more different than Mr. Trump’s first trial a year ago. Then, the House tried to make its case around an esoteric plot to pressure Ukraine to smear Mr. Biden, and it failed largely on party lines.

But over five days this week, the House managers put forward in harrowing detail an account of a horror that had played out in plain sight. Using graphic video and sophisticated visual aids, they made clearer than ever before how close the armed mob had come to a dangerous confrontation with Vice President Mike Pence and the members of the House and the Senate.

All of it, the prosecutors argued, was the doing of Mr. Trump, who spread lies that the election had been “stolen” from him, cultivated outrage among his followers, encouraged violence, tried to pressure state election officials to overturn democratically decided results and finally assembled and unleashed a mob of his supporters — who openly planned a bloody last stand — to “stop the steal.” With no signs he was remorseful, they warned he could ignite a repeat if allowed to seek office again.

“If that is not ground for conviction, if that is not a high crime and misdemeanor against the Republic and the United States of America, then nothing is,” Representative Jamie Raskin, Democrat of Maryland and the lead manager, said as he summed up his case. “President Trump must be convicted, for the safety and democracy of our people.”

After stumbling out of the gate earlier in the week with meandering presentations, Mr. Trump’s legal team delivered the president a highly combative and exceedingly brief defense on Friday. Calling the House’s charge a “preposterous and monstrous lie,” they insisted over just three hours that the former president was a “law and order”-loving leader who never meant for his followers to take the words “fight like hell” literally, and could not have foreseen the violence that followed.

“They were not trying a case,” Michael T. van der Veen, a member of the hastily assembled legal team, said of Democrats in his own closing remarks. “They were telling a political tale, a fable, and a patently false one at that.”

They also offered more technical arguments aimed at giving Republicans refuge for acquittal, arguing that it was not constitutional for the Senate to try a former president and that Mr. Trump’s election lies and bellicose words to his supporters could not be deemed incitement because the First Amendment protected his right to speak freely.

The seven Republicans who rejected those arguments in favor of conviction were an ideologically diverse group at various stages of their political careers. Mr. Burr and Mr. Toomey plan to retire next year. Mr. Cassidy, Ms. Collins and Mr. Sasse were just re-elected, and Mr. Romney and Ms. Murkowski are among Mr. Trump’s most durable Republican critics.

They appeared to draw strength from one another. Shortly before the vote, Mr. Cassidy walked a note to Mr. Burr. It read, “I am a yes,” he said later. Mr. Burr nodded back at him.

Ms. Murkowski, who faces re-election next year in a state Mr. Trump won twice, said afterward she would not let her vote be “devalued by whether or not I feel that this is helpful for my political ambitions.”

“This is not about me,” she told reporters. “This is really about what we stand for, and if I can’t say what I believe, what our president should stand for, then why should I ask Alaskans to stand with me?”

After the attack and Republicans’ loss of the Senate, there had been a brief window in which it seemed as if the outcome might be different. Mr. McConnell privately told advisers that an impeachment conviction might be the only way to purge Mr. Trump from the party after four tumultuous years, and his openness to finding him guilty held out the possibility that a coalition of Republicans might follow his lead.

But by the time the proceeding began, with Mr. Biden already in office, the party’s rank and file in Congress had made clear that Mr. Trump still had far too strong a pull among their voters to engage in a head-on fight. As the former president threatened to back primary challengers to the House Republicans who voted to impeach him, state parties across the country lined up votes to censure them or call for their resignations.

Emily Cochrane, Luke Broadwater and Jonathan Martin contributed reporting.