WASHINGTON (NewsNation Now) — House Republican Leader Kevin McCarthy said Tuesday that he won’t support a proposal to form an independent, bipartisan commission to study the deadly Jan. 6 riots at the U.S. Capitol, almost certainly eroding GOP support ahead of a vote and positioning his party as opposed to investigations of the attack.

McCarthy said he wanted the new panel to look beyond the violent uprising by supporters loyal to former President Donald Trump, who were trying to stop the certification of President Joe Biden’s election. McCarthy pushed to have the new commission also investigate other groups, namely the Black Lives Matter groups that protested police brutality and racial injustice in the aftermath of the death of George Floyd.

He said that given the “shortsighted scope that does not examine interrelated forms of political violence in America, I cannot support this legislation.”

McCarthy’s opposition all but ensures this week’s vote will have less Republican support in the House, and dims its chances in the evenly divided Senate — threatening the bipartisan commission’s chances as Democrats say it is essential to reckoning with the violent attack that interrupted the electoral count.

House Speaker Nancy Pelosi told reporters on Capitol Hill: “It’s disappointing, but not surprising, that the cowardice on the part of some on the Republican side — not to want to find the truth.”

Ahead of a vote, the Biden administration said it supports the legislation to “investigate the facts” on the “unprecedented” attack.

“The Nation deserves such a full and fair accounting to prevent future violence and strengthen the security and resilience of our democratic institutions,” the White House said in a statement of administration policy.

The GOP leader’s dissent comes as some other Republicans minimize the severity of the attack. Numerous Republican lawmakers joined McCarthy in speaking against the measure early Tuesday during a meeting of House Republicans, according to one Republican familiar with the private session and granted anonymity to discuss it.

Rep. John Katko, the top Republican on the Homeland Security Committee, who drafted the legislation with the panel’s Democratic Chairman Rep. Bennie Thompson of Mississippi, defended the proposed commission at the closed-door meeting Tuesday, the person said.

“I recognize there are differing views on this issue, which is an inherent part of the legislative process and not something I take personally,” Katko, R-N.Y., said in a statement. “However, as the Republican leader of the Homeland Security Committee, I feel a deep obligation to get the answers U.S. Capitol Police and Americans deserve and ensure an attack on the heart of our democracy never happens again.”

But House Majority Leader Steny Hoyer said he still expects a “significant number” of Republicans to vote in favor of the legislation.

At a House hearing last week investigating the siege, one member denied there was an insurrection at all while another said a woman who was shot and killed by police while trying to break into the House chamber was “executed.” Many other Republicans have said Democrats should focus on the violence in cities instead.

And it comes as some have suggested McCarthy himself could be subpoenaed by the panel because he talked to Trump as the insurrection was happening. Wyoming Rep. Liz Cheney, who was booted out of GOP leadership last week for her criticism of Trump’s unfounded claims about the election being stolen. Cheney has supported the commission.

As proposed, the bill does not suggest the commission investigate the riots in the aftermath of Floyd’s death, as Katko’s colleagues had suggested. But it does create a panel that is evenly split between Republicans and Democrats and gives them equal subpoena power — a change from Pelosi’s original proposal, which had more Democrats sitting on the commission and weighted subpoena power.

Like Cheney, Katko was one of 10 Republicans who voted to impeach Trump after the insurrection for telling his supporters that day to “fight like hell” to overturn his defeat. The Senate later acquitted him.

Modeled after the investigation into the 9/11 terrorist attacks, the bill would establish an independent 10-member commission that would make recommendations for preventing another insurrection. The panel would have to issue a final report by Dec. 31.

The bill’s path forward is uncertain in the 50-50 Senate, where Republicans have been quiet on the commission in recent weeks and several have said they don’t think it’s needed. Senate Republican Leader Mitch McConnell objected to the initial proposal by Pelosi, saying it should also investigate last summer’s riots.

The Associated Press contributed to this article: Reporting by Mary Clare Jalonick, Alan Fram and Lisa Mascaro.