A sharply divided House voted on Wednesday to create an independent commission to investigate the Jan. 6 Capitol assault, overcoming opposition from Republicans determined to stop a high-profile accounting of the deadly pro-Trump riot.
But even as the legislation passed the House, top Republicans locked arms in an effort to doom it in the Senate and shield former President Donald J. Trump and their party from fresh scrutiny of their roles in the events of that day.
The final vote in the House, 252 to 175, with four-fifths of Republicans opposed, pointed to the difficult path ahead.
The vote came hours after Senator Mitch McConnell, the minority leader, declared his opposition to the plan. Mr. McConnell had said just a day earlier that he was open to voting for it, and had previously been vocal both in denouncing Mr. Trump’s role in instigating the assault and in decrying the effort by some Republicans on Jan. 6 to block certification of the election results.
His reversal reflected broader efforts by the party to put the assault on the Capitol behind them in political terms, or recast the rioting as a peaceful protest, amid pressure from Mr. Trump and concerns about the issue dogging them in the 2022 elections.
Proponents, including 35 Republicans, hailed the move to establish the commission as an ethical and practical necessity to fully understand the most violent attack on Congress in two centuries and the election lies from Mr. Trump that fueled it. Modeled after the body that studied the Sept. 11, 2001, terrorist attack, the 10-person commission would take an inquiry out of the halls of Congress and deliver findings by Dec. 31.
“I was on the Capitol floor, the speaker was in the chair and a howling mob attacked the United States Capitol,” Representative Zoe Lofgren, Democrat of California and the chairwoman of a committee already studying the attack, said in an animated appeal before the vote. She reminded colleagues of the “pounding on the doors” and “maimed police officers.”
“We need to get to the bottom of this to not just understand what happened leading up to the 6th, but how to prevent that from happening again — how to protect the oldest democracy in the world in the future,” Ms. Lofgren said.
Among the Republicans voting in favor of the commission were a familiar group of moderates and stalwart critics of Mr. Trump, but also several longer-serving members from safe conservative districts who were rattled by the attack. The most notable was Representative Liz Cheney of Wyoming, who was run out of the party leadership last week because she refused to stop criticizing Mr. Trump for his attempts to overturn the election.
But the prospects for Senate passage dimmed substantially after Mr. McConnell joined his House counterpart, Representative Kevin McCarthy of California, and Mr. Trump in panning the proposal crafted by Democrats and a moderate House Republican as overly partisan and duplicative of ongoing Justice Department criminal prosecutions and narrow congressional investigations.
“After careful consideration, I’ve made the decision to oppose the House Democrats’ slanted and unbalanced proposal for another commission to study the events of Jan. 6,” Mr. McConnell said on the Senate floor.
Senator Chuck Schumer, Democrat of New York and the majority leader, vowed to put it up for a vote there in the coming weeks to force Republicans to choose.
“An independent commission can be the antidote to the poisonous mistruths that continue to spread about Jan. 6, and that is what our founding fathers believed in,” he said. “The American people will see for themselves whether our Republican friends stand on the side of truth or on the side of Donald Trump’s big lie.”