WASHINGTON — The House committee investigating the Jan. 6, 2021, attack on the Capitol plans to use the testimony of former President Donald J. Trump’s own campaign manager against him on Monday as it lays out evidence that Mr. Trump knowingly spread the lie that the 2020 election was stolen from him in an attempt to overturn his defeat.
The committee plans to call Bill Stepien, the final chairman of Mr. Trump’s campaign, who is expected to be asked to detail what the campaign and the former president himself knew about his fictitious claims of widespread election fraud. Those claims will be the focus of the second in a series of hearings the panel is holding this month to reveal the findings of its sprawling investigation.
After an explosive first hearing last week in prime time, leaders of the committee are aiming to keep up a steady stream of revelations about the magnitude of Mr. Trump’s plot to overturn the election and how it sowed the seeds of the violent siege of the Capitol by his supporters last year.
On Monday, they plan to describe the origin and spread of Mr. Trump’s election lies, including the former president’s refusal to listen to advisers who told him that he had lost and that there was no evidence of widespread irregularities that could change the outcome. Then they plan on demonstrating the chaos those falsehoods caused throughout several states, ultimately resulting in the riot.
A second panel of witnesses will include Byung J. Pak, a former U.S. attorney in Atlanta who resigned abruptly after refusing to say that widespread voter fraud had been found in Georgia.
According to an internal memo made public as part of a court case, the Trump campaign knew as early as November that its outlandish fraud claims were false. Last week, the panel showed videotaped testimony of his top advisers and even the attorney general at the time, William P. Barr, saying that they had told Mr. Trump and top White House officials as much.
Mr. Stepien was present for key conversations about what the data showed about Mr. Trump’s chances of succeeding in an effort to win swing states, beginning on election night. He was part of a meeting with Mr. Trump on Nov. 7, 2020, just after the election had been called by television networks in favor of President Joseph R. Biden Jr., in which he told Mr. Trump of the exceedingly low odds of success with his challenges.
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Mr. Trump, urged on by his lawyer, Rudolph W. Giuliani, wanted to press forward anyway.
Mr. Stepien, who rarely speaks in public, is appearing under subpoena, raising questions about how willing a witness he will be against Mr. Trump.
Mr. Stepien is currently serving as an adviser to Harriet Hageman, a Republican endorsed by Mr. Trump who is mounting a primary challenge to Representative Liz Cheney, Republican of Wyoming and the panel’s vice chairwoman, setting up a potentially adversarial dynamic for his questioning on Monday.
The Jan. 6 committee suggested in a letter sent to Mr. Stepien that it had evidence that he was aware that the campaign was raising money by making false claims about election fraud.
“As manager of the Trump 2020 re-election campaign, you oversaw all aspects of the campaign,” the letter said. “You then supervised the conversion of the Trump presidential campaign to an effort focused on ‘Stop the Steal’ messaging and related fund-raising. That messaging included the promotion of certain false claims related to voting machines despite an internal campaign memo in which campaign staff determined that such claims were false.”
Mr. Stepien will appear alongside Chris Stirewalt, the former political editor at Fox News who was fired after correctly calling the 2020 president election in Arizona for Mr. Biden, a move that angered Mr. Trump.
The second part of the hearing will turn to the reverberations of Mr. Trump’s false claims around the country, particularly in competitive states. Along with Mr. Pak, who resigned after learning that Mr. Trump wanted to fire him for rejecting claims of rampant voter fraud in Georgia, the panel is scheduled to hear from Al Schmidt, a Republican former city commissioner in Philadelphia who also stood up to Mr. Trump’s lies. Benjamin Ginsberg, a Republican election lawyer who served as the national counsel to George W. Bush’s presidential campaign and played a central role in the Florida recount of 2000, is also slated to appear.
Monday’s lineup of witnesses suggests that the committee wants to chart the impact Mr. Trump’s lies had in conservative media and in various states, as well as contrasting the baseless nature of Mr. Trump’s claims with legitimate legal challenges from Republican campaigns of the past.
Shortly after the election, as ballots were still being counted, the top data expert in Mr. Trump’s re-election campaign told him bluntly that he was going to lose.
In the weeks that followed, as Mr. Trump continued to insist that he had won, a senior Justice Department official told him repeatedly that his claims of widespread voting fraud were meritless, ultimately warning him that they would “hurt the country.”
Those concerns were echoed by the top White House lawyer, who told the president that he would be entering into a “murder-suicide pact” if he continued to pursue extreme plans to try to invalidate the results of the 2020 election.
Last week, the Jan. 6 panel played video of an interview showing Mr. Barr testifying that he knew the president’s claims were false, and told him so on three occasions.
“I told the president it was bullshit,” Mr. Barr is heard telling the committee’s investigators. “I didn’t want to be a part of it.”
Committee members previewed some of the evidence they plan to present at Monday’s hearing during television news interviews Sunday.
“Former President Trump was told by multiple people — it should have been abundantly clear — that there was no evidence that showed the election was stolen, and he ignored that,” Representative Elaine Luria, Democrat of Virginia and a member of the committee, said on NBC’s “Meet The Press.”
Representative Adam Kinzinger, Republican of Illinois, drew a contrast between those close to Mr. Trump who told him the truth and the “yes people” who encouraged his fantasy of a stolen election in order to please him.
“If you truly believe the election was stolen, then if the president truly believed that, he’s not mentally capable to be president,” Mr. Kinzinger said on CBS’s “Face The Nation,” adding: “I think he didn’t believe it. I think the people around him didn’t believe it. This was all about keeping power against the will of the American people.”