On Thursday, President Biden slammed former President Donald J. Trump and his allies for holding “a dagger at the throat of America” by promoting lies and violence as the nation’s capital splintered into sparring camps a year after the Jan. 6 mob assault on Congress.

Mr. Biden used the anniversary of the Capitol siege to condemn Mr. Trump for waging a “undemocratic” and “un-American” campaign against the legitimacy of the election system, much like autocrats and dictators do to avoid admitting defeat.

“The former president of the United States of America has created and spread a web of lies about the 2020 election,” Mr. Biden said, standing in the National Statuary Hall, which had been invaded by throngs of Trump supporters a year ago. “He’s done so because he values power over principle, because he sees his own interests as more important than his country’s interests and America’s interests, and because his bruised ego matters more to him than our democracy or our Constitution. He can’t accept he lost.”

The president’s speech kicked off a commemoration that, rather than demonstrating American unity in the face of threats to democracy, only highlighted how divided the country remains a year after rioters armed with hockey sticks, baseball bats, crutches, flagpoles, fire extinguishers, bear spray, and stolen police batons broke into the Capitol to disrupt the counting of the Electoral College votes ratifying Mr. Trump’s defeat.

Democrats marked the anniversary with a day of events, including speeches, personal testimony, a panel of historians, videos, moments of silence, and a candlelight vigil, while Republicans largely stayed away and refused to participate.

There were no Republican senators on the floor for a session of remarks commemorating that day. Representative Liz Cheney of Wyoming, one of Mr. Trump’s harshest critics, was the only elected member of her party to join a moment of silence in the House chamber, bringing her father, former Vice President Dick Cheney, with her.

The various approaches to the anniversary demonstrated that January 6 has become yet another barometer of America’s partisan divide. Democrats see the Capitol storming as an existential attack on the Constitution unlike any other in modern history. Most Republicans would rather focus on something else, with some convinced that Democrats are using it against them, while others are afraid of upsetting Mr. Trump, who continues to dominate the party.

Feelings remained raw on Capitol Hill, a post-traumatic stress disorder hotspot that has yet to fully recover from the psychological and political scars of an assault that resulted in at least seven deaths and 150 police officers being injured. More than the usual squabbles over legislation, the legacy of Jan. 6 has widened the toxic schism between members and staff aides on opposing sides of the aisle.

Right-wing groups’ online chatter about celebrations and rallies to protest what they call the persecution of the hundreds of arrested rioters did not translate into large-scale events, and the day passed peacefully in Washington.

Democrats, on the other hand, recalled the fear and dread of that day, when lawmakers were rushed out of their chambers by overwhelmed police officers who couldn’t keep rioters chanting “Hang Mike Pence” and searching for California Speaker Nancy Pelosi.

Senator Patrick J. Leahy of Vermont took a breather to gather his thoughts as he remembered the police officer who took his arm and vowed to protect him. Representative Colin Allred of Texas, a powerfully built former National Football League player, described shedding his coat and bracing himself for the prospect of having to physically protect his colleagues.

Ms. Pelosi hosted a discussion led by Carla Hayden, the Librarian of Congress, with historians Doris Kearns Goodwin and Jon Meacham who talked about other moments of peril, like the years leading up to the Civil War. The panel was introduced with a video specially produced by the cast of “Hamilton,” performing a song from the hit musical about the founding of the country, to highlight the significance of the event.