WASHINGTON — The House committee investigating the Jan. 6, 2021, attack on the Capitol plans to open a landmark series of public hearings on Thursday night by playing previously unreleased video of former President Donald J. Trump’s top aides and family members testifying before its staff, as well as footage revealing the role of the Proud Boys, a right-wing extremist group, in the assault.
Committee aides say the evidence will show that Mr. Trump was at the center of a “coordinated, multistep effort to overturn the results of the 2020 election” that resulted in a mob of his supporters storming the halls of Congress and disrupting the official electoral count that is a pivotal step in the peaceful transfer of presidential power.
The 8 p.m. hearing is the first in a series of six planned for this month, during which the panel plans to lay out for Americans the full magnitude and significance of Mr. Trump’s systematic drive to invalidate the 2020 election and remain in power.
“We’ll demonstrate the multipronged effort to overturn a presidential election, how one strategy to subvert the election led to another, culminating in a violent attack on our democracy,” said Representative Adam B. Schiff, Democrat of California and a member of the committee. “It’s an important story, and one that must be told to ensure it never happens again.”
The prime-time hearing will feature live testimony from a documentary filmmaker, Nick Quested, who was embedded with the Proud Boys during the attack, and a Capitol Police officer, Caroline Edwards, who was injured as rioters breached barricades and stormed into the building.
The committee also plans to present what aides called a small but “meaningful” portion of the recorded interviews its investigators conducted with more than 1,000 witnesses, including senior Trump White House officials, campaign officials and Mr. Trump’s family members.
Mr. Trump’s elder daughter, Ivanka Trump, his son-in-law Jared Kushner and his son Donald Trump Jr. are among the high-profile witnesses who have testified before the panel.
Mr. Quested, a British documentarian who has worked in war zones such as Afghanistan, spent a good deal of the postelection period filming members of the Proud Boys, including the group’s former chairman, Enrique Tarrio, who has been charged with seditious conspiracy in connection with the Capitol riot. Mr. Quested accompanied the Proud Boys to pro-Trump rallies in Washington in November and December 2020 and was on the ground with members of the group on Jan. 6, when several played a crucial role in breaching the Capitol.
Ms. Edwards, a well-respected Capitol Police officer, is believed to be the first officer injured in the attack, when she suffered a concussion during an assault at a barricade at the base of Capitol Hill. A man who has been charged with taking part in the assault, Ryan Samsel, told the F.B.I. during an interview more than a year ago that just before he approached the barricade, a high-ranking member of the Proud Boys, Joseph Biggs, had encouraged him to confront the police.
Other officers around the building recall hearing Officer Edwards calling for help over the radio — one of the first signs that mob violence was beginning to overrun the police presence. Months after the attack, she continued to have fainting spells believed to be connected to her injuries.
Representative Bennie Thompson, Democrat of Mississippi and the committee chairman, and Representative Liz Cheney, Republican of Wyoming and the vice chairwoman, are expected to lead the presentation of the panel’s evidence and question the witnesses.
The session will kick off an ambitious effort by the committee, which was formed in July after Republicans blocked the creation of a nonpartisan commission to investigate the attack, to lay out for Americans the full story of an unprecedented assault on U.S. democracy that led to a deadly riot, an impeachment and a crisis of confidence in the political system that continues to reverberate.
The hearings are unfolding five months before midterm elections in which the Democrats’ majority is at stake, at a time when they are eager to draw a sharp contrast between themselves and the Republicans who enabled and embraced Mr. Trump, including the members of Congress who abetted his efforts to overturn the 2020 election.
Republicans are planning countermessaging that began on Thursday morning, when the party’s House leaders took turns at a news conference on Capitol Hill bashing the panel’s work as “illegitimate” and a “sham.”
They argued that the committee was out of touch with the pocketbook concerns of many Americans.
“Is Nancy Pelosi going to hold a prime-time hearing on inflation?” Representative Steve Scalise of Louisiana, the No. 2 Republican, said. “I’d sure like to see that. I think a lot of Americans would. Is Nancy Pelosi going to hold a prime-time hearing on lowering gas prices? They’ve refused to so far.”