WASHINGTON — For most of his first year in office, President Biden has bet that he could move the country past the divisiveness of his predecessor by restoring a sense of normalcy to the White House, practicing the traditional brand of politics he learned over decades in the Senate and as vice president — and largely ignoring the man he refers to as “the former guy.”

It didn’t work.

So on Thursday, Mr. Biden put aside his hopes of no longer having to engage directly with Donald J. Trump and went aggressively at him, using an impassioned speech in the Capitol to make clear the urgent necessity of confronting Mr. Trump — and Trumpism.

“We saw it with our own eyes. Rioters menaced these halls, threatening the life of the speaker of the house, literally erecting gallows to hang the vice president of the United States of America,” Mr. Biden said from National Statuary Hall.

“What did we not see?” he continued. “We didn’t see a former president who had just rallied the mob to attack, sitting in the private dining room off the Oval Office in the White House, watching it all on television and doing nothing for hours as police were assaulted, lives at risk, the nation’s capitol under siege.”

Later, Mr. Biden was even more blunt, even as he refused to utter Mr. Trump’s name. “He was just looking for an excuse, a pretext, to cover for the truth,” he said of Mr. Trump’s lies about election fraud. “He’s not just a former president. He’s a defeated former president.”

The extraordinary moment, in which a sitting president accused his predecessor of holding “a dagger at the throat of America, at American democracy,” marked a sharp pivot in Mr. Biden’s strategy for dealing with Mr. Trump and his continuing promotion of the baseless assertion that the 2020 election was marred by fraud.

The president’s speech tacitly acknowledged that his predecessor, far from fading away, remains the most potent force in Republican politics and a credible rival to Mr. Biden in 2024. And for Mr. Biden, who throughout the last year has articulated the importance of promoting democracy over autocracy around the world, it also signaled his willingness to confront more directly the challenges Mr. Trump poses to democratic values at home, which have shown little sign of dissipating in the year since a violent mob tried to block the certification of Mr. Biden’s election victory.

The approach has its risks, not least in providing Mr. Trump with better opportunities to hit Mr. Biden with broadsides of his own — an opening that Mr. Trump seized on Thursday with a series of angry statements accusing the president of supporting “open borders,” “unconstitutional mandates” and “corrupt elections.”

But continuing to ignore his predecessor carries real peril for Mr. Biden as well. Recent polling suggests that millions of Americans are at least somewhat willing to tolerate or support political violence against partisan opponents.

Republican-controlled states are considering or enacting restrictions on voting rights. Supporters of Mr. Trump are seeking to control the machinery of elections in key states, potentially giving them the power to block an outcome they oppose. Substantial majorities of Republicans in polls say they believe the results of the 2020 election were illegitimate.

Mr. Trump’s influence over the Republican Party remains strong — he is trying to be its de facto kingmaker and he is polling as its front-runner for the 2024 presidential election. His false statements on election fraud continue to divide Americans.

Last month, the two presidents shared a rare occurrence: commending each other. In an effort to address vaccine hesitancy among many Trump supporters — unvaccinated Americans are disproportionally Republican — Mr. Biden praised the previous administration’s work on coronavirus vaccines, prompting Mr. Trump to express gratitude.

Since his inauguration, Mr. Biden has repeatedly condemned the violent assault on the Capitol and has even criticized Mr. Trump by name on a few occasions. Yet before Thursday, he had never as president taken such a direct, aggressive tone against Mr. Trump and his falsehoods, or the Republicans who have enabled him.

“He values power over principle,” Mr. Biden said of Mr. Trump. “Because he sees his own interests as more important than his country’s interest, and America’s interest. And because his bruised ego matters more to him than our democracy or our constitution.”

Frank Luntz, a Republican strategist, said returning to a contentious tit-for-tat would only alienate Trump supporters the administration was hoping to vaccinate.

“We can save millions of lives globally but when we tear each other apart like we did on Jan. 6, the damage can be irreparable,” Mr. Luntz said.

The president said “some courageous men and women” were trying to uphold the principles of the Republican Party, but “too many others are transforming that party into something else.”

Mr. Biden said he was willing to work with Republicans in Congress “who support the rule of law and not the rule of a single man.”

Michael Chertoff, the former homeland security secretary under President George W. Bush and a Republican, said the shift by Mr. Biden was necessary because Mr. Trump’s false statements about the 2020 election and the assault on the Capitol amounted to a national security threat. The F.B.I. and the Homeland Security Department have issued multiple assessments concluding that such misinformation has emboldened domestic extremists to commit violence.

“Given Trump’s ego, it’s absolutely appropriate to look him in the eye and say, ‘I know what you did, it’s not appropriate and it’s not going to happen again,’” Mr. Chertoff said. “It was necessary for the president to show I am not shrinking from calling out what is going on.”

David Axelrod, a former top adviser to former President Barack Obama, said Mr. Biden should maintain the same tone in the future regarding Mr. Trump.

“Going after Trump, who remains deeply unpopular outside his base, could be smart politics, especially if it draws him back into the fray,” Mr. Axelrod said, adding that there was a need to confront the ideology that fueled attack on the Capitol. “Hard to take that on without confronting the author and chief purveyor of the lie.”

Even as Mr. Biden confronted Mr. Trump, there is little sign the address will change the behavior of Republicans beholden to the former president and reluctant to cooperate with Mr. Biden.

Representative Kevin McCarthy of California, the House Republican leader, said in the days after the riot that Mr. Trump “bears responsibility” for the violence, only to later travel to Mar-a-Lago to preserve his relationship with the former president. Senator Mitch McConnell, Republican of Kentucky and the minority leader, has been more forceful in his condemnations of the former president, but some longtime conservatives are showing increasing anxiety over Mr. Trump’s continued grip on the party.

Karl Rove, a former senior adviser to President George W. Bush, criticized “Republicans who for a year have excused the actions of the rioters who stormed the Capitol” in an opinion piece this week in The Wall Street Journal.

Mr. Biden, with a slim majority in Congress, is struggling to unite his party behind his priorities: advancing a climate and social-spending package bill, as well as federal voting rights legislation. The president’s approval ratings have been low, in part because of rising inflation and the pandemic, making the passage of his agenda even more crucial ahead of the midterm elections.

Pressed by reporters after his address over whether his remarks would only deepen divisions in America, Mr. Biden said he did not intend to create “a contemporary political battle” with Mr. Trump.

But he said candor was vital to moving forward.

“The way you have to heal, you have to recognize the extent of the wound,” Mr. Biden said. “You can’t pretend. This is serious stuff.”