Some Republicans have turned to conspiracy theories and misinformation to explain the Capitol attack.

Immediately after the attack on the U.S. Capitol, all corners of the political spectrum repudiated the mob of President Trump’s supporters. Yet within days, prominent Republicans, party officials, conservative media voices and rank-and-file voters began making a rhetorical shift to try to downplay the group’s violent actions.

In one of the ultimate don’t-believe-your-eyes moments of the Trump era, these Republicans have retreated to the ranks of misinformation, claiming it was Black Lives Matter protesters and far-left groups like antifa who stormed the Capitol — in spite of the pro-Trump flags and QAnon symbology in the crowd. Others have argued that the attack was no worse than the rioting and looting in cities during the Black Lives Matter movement, often exaggerating the unrest last summer while minimizing a mob’s attempt to overturn an election.

The shift is revealing about how conspiracy theories, deflection and political incentives play off one another in Mr. Trump’s G.O.P. For a brief time, Republican officials seemed perhaps open to grappling with what their party’s leader had wrought — violence in the name of their Electoral College fight. But any window of reflection now seems to be closing as Republicans try to pass blame and to compare last summer’s lawlessness, which was condemned by Democrats, to an attack on Congress, which was inspired by Mr. Trump.

“The violence at the Capitol was shameful,” Rudolph W. Giuliani, the president’s lawyer, tweeted at 6:55 a.m. the morning after the attack. “Our movement values respect for law and order and for the police.” But now, in a new video titled “What Really Happened on January 6th?” Mr. Giuliani is among those who are back to emphasizing conspiracy theories.

“The riot was preplanned,” said Mr. Giuliani, the former mayor of New York City. “This was an attempt to slander Trump.” He added, “The evidence is coming out.”

Suzanne Doherty, 67, who traveled from Michigan to be in Washington on Jan. 6 to support Mr. Trump, came away feeling confused and depressed over the invasion of the Capitol and not trusting the images of the mob.

“I heard that on antifa websites, people were invited to go to the rally and dress up like Trump supporters, but I’m not sure what to believe anymore,” she said. “There were people there only to wreak havoc. All I know is that there was a whole gamut of people there, but the rioters were not us. Maybe they were antifa. Maybe they were B.L.M. Maybe they were extreme right militants.”

The conjecture that the mob was infiltrated by Black Lives Matter and antifa has been metastasizing from the dark corners of the pro-Trump internet to the floors of Congress and the Republican base, even as law enforcement officials say there is no evidence to support it. The authorities are now flagging threats of violence and rioting leading up to President-elect Joseph R. Biden Jr.’s inauguration.

That has not stopped Republican lawmakers and some of their constituents from pushing these narratives to defend Mr. Trump.