WASHINGTON — The House committee investigating the Jan. 6 attack on the Capitol was set 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 said the evidence would 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, a pivotal step in the peaceful transfer of presidential power.

The 8 p.m. hearing was the first in a series of at least six planned for this month, during which the panel planned 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. In doing so, its leaders said they hoped to force the nation to grapple with the truth of a dark chapter in its history, one that is still reverberating.

“Our democracy remains in danger,” said Representative Bennie Thompson, Democrat of Mississippi and the chairman of the committee, in opening remarks provided in advance of the session. “Jan. 6 and the lies that led to insurrection have put two and a half centuries of constitutional democracy at risk. The world is watching what we do here.”

The session was to kick off an ambitious effort by the committee, made up of seven Democrats and two Republicans, to lay out an aggressive and unprecedented effort by a sitting president to overturn an election, one that tested the guardrails of American democracy at every turn. Mr. Trump and his allies challenged President Biden’s victory in the courts, at state houses and, finally, in the streets.

When one plan to keep Mr. Trump in power failed, they shifted to another. They put forward slates of pro-Trump electors in states won by Mr. Biden; they explored the seizure of voting machines; they targeted the removal of the acting attorney general; and, finally, when all else failed, they began amassing a mob to march to the Capitol to pressure Mr. Trump’s own vice president to go along with the plan.

Given the gravity of the threat to democracy, the members of the committee see themselves as carrying out a critical and historic function, much as landmark fact-finding committees did before them, investigating the attack of Pearl Harbor in 1941, the assassination of President John F. Kennedy in 1963, the Watergate scandal in 1973 and the terrorist attacks of Sept. 11, 2001.

The first hearing, unfolding in prime time in a stately hearing room on Capitol Hill, was set to 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 planned 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 much 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 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.

Officer Edwards, a well-respected member of the force, 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.

Mr. Thompson and Representative Liz Cheney, Republican of Wyoming and the vice chairwoman, were expected to lead the presentation of the panel’s evidence and question the witnesses.

The committee was formed in July after Republicans blocked the creation of a nonpartisan commission to investigate the attack. The hearings to lay out its findings 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 and began that effort 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?” said Representative Steve Scalise of Louisiana, the No. 2 Republican. “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.”

Other hearings were expected to focus on various aspects of the committee’s investigation, including Mr. Trump’s promotion of the lie that the election had been stolen, despite being told his claims were false; his attempts to misuse the Justice Department to help him cling to power; a pressure campaign on Vice President Mike Pence to throw out legitimate electoral votes for Mr. Biden; the way the mob was assembled and how it descended on Washington on Jan. 6; and the fact that Mr. Trump did nothing to stop the violence for more than three hours while the assault was underway.

The Jan. 6 panel has not yet committed to the full slate of witnesses for the six televised hearings, and it is still discussing the possibility of public testimony with several prominent Trump-era officials.

Among the witnesses the committee has formally approached to testify next week are Jeffrey A. Rosen, the former acting attorney general, and Richard P. Donoghue, the former acting deputy attorney general, according to two people briefed on the matter.

Mr. Rosen and Mr. Donoghue have told multiple congressional committees that Mr. Trump and his allies pressured the department to falsely say that it had found voter fraud and to use its power to undo the election results. Last May, Mr. Rosen took part in a public hearing of the House Oversight and Reform Committee on events leading up to the assault on the Capitol.

Alan Feuer and Katie Benner contributed reporting.