Senate Minority Leader Mitch McConnell (R-Ky.) said Wednesday morning that he will oppose a commission to investigate the deadly Jan. 6 attack on the Capitol, calling the bipartisan proposal slanted and unbalanced.

The House is set to vote later Wednesday on a bill to create the commission. House Minority Leader Kevin McCarthy (R-Calif.) also opposes the plan .

The commission would consist of 10 independent experts who would investigate the causes of the Capitol riot and make recommendations to prevent it from happening again.

McConnell claimed House Democrats negotiated in bad faith on the basic parameters of the commission. Thats not true. House Speaker Nancy Pelosi (D-Calif.) agreed to nearly all of Republicans demands for the panel, including equal party representation, shared subpoena power, and mandating that a report be released by the end of the year.

McConnell had previously complained that the commission wouldnt focus on political violence related to Black Lives Matter protests last year, but this week seemed to drop that objection in favor of one about a commission duplicating investigations by federal law enforcement and congressional committees.

There is, has been, and there will continue to be no shortage no shortage of robust investigations by two separate branches of the federal government, McConnell said Wednesday. Its not at all clear what new facts or additional investigation yet another commission could actually lay on top of existing efforts by law enforcement and Congress.

McConnell previously said he would support a commission modeled on the one Congress established after the Sept. 11 terrorist attacks. That commission also complemented the investigatory work of law enforcement and congressional committees , where lawmakers tend to grandstand and avoid legitimate lines of inquiry.

The leaders of the 9/11 Commission, former New Jersey Gov. Tom Kean and former Indiana Rep. Lee Hamilton, issued a statement on Wednesday urging the House to pass the bill establishing a Jan. 6 commission. They called on Congress to set aside partisan politics and come together as Americans in common pursuit of truth and justice.

McConnells opposition makes setting up a commission much more difficult, suggesting it may be the first bill Republicans filibuster in this Congress.

GOP senators who previously expressed interest in supporting setting up the panel are already changing their tune. Sen. Mike Rounds (R-S.D.), for example, told reporters on Tuesday that he was interested in setting up a commission narrowly focused on the events of Jan. 6. However, on Wednesday, after hearing directly from McCarthy, Rounds said he could no longer support the effort because it wasnt bipartisan in nature.

Other Republican senators complained about the composition of the commission, noting the Democratic-appointed chair would have the sole power to hire committee staff. But that feature is modeled after the bipartisan 9/11 commission, which the Senate approved in 2002 with a 90-8 vote (McConnell voted yes on it).

Sen. Bill Cassidy (R-La.), meanwhile, said he disagreed with McConnells comments about the nature of the commission, adding that he was inclined to support it. Cassidy was one of seven GOP senators who voted to convict Trump over the insurrection earlier this year.

Senate Majority Leader Chuck Schumer (D-N.Y.) said Tuesday that he will hold a vote on the bill to create a commission even if Republicans vow to oppose it.

Republicans can let their constituents know, are they on the side of truth [or do they] want to cover up for the insurrectionists and for Donald Trump ? he said.

Elise Foley contributed reporting.

CORRECTION: This article previously called Tom Kean the former governor of New Hampshire. He is the former governor of New Jersey.