The Democrat who negotiated the bipartisan bill creating a commission to investigate the Capitol attack of Jan. 6 suggested Wednesday that GOP leaders had endorsed the legislation, only to backtrack later for political reasons.

Rep. Bennie Thompson (D-Miss.), chairman of the Homeland Security Committee, spent months in fragile negotiations with Rep. John Katko (N.Y.), the panel's senior Republican, to seal a deal on the proposal, which would create an outside panel, modeled on the 9/11 Commission, to examine the violent rampage of Jan. 6.

Finalized Friday, the bill is set to hit the floor Wednesday evening.

Thompson said Republican leaders were actively involved throughout those talks, leaving the negotiators to think they were ready to sign off on the final product.

"From the beginning, leadership on the Republican and Democratic side was kept informed as to what John and I [were] involved in. But more importantly, leadership weighed in and made suggestions as to how it could be improved, what we needed to take out," Thompson told reporters in the Capitol.

"The ranking member and I spent many hours working back and forth getting to this point of accepting the document," he added. "And in principle we thought we actually had agreement on all sides."

Yet the debate over the bipartisan bill took a quick partisan turn Tuesday, when House Minority Leader Kevin McCarthy (R-Calif.) announced his opposition. McCarthy has demanded that the outside panel not limit its investigation to the Jan. 6 attack, which was carried out by supporters of former President Trump, but expand its lens to include other incidents of political violence associated with liberal groups.

“Given the political misdirections that have marred this process, given the now duplicative and potentially counterproductive nature of this effort, and given the Speaker’s shortsighted scope that does not examine interrelated forms of political violence in America, I cannot support this legislation,” he said Tuesday in a statement.

Initially, GOP leaders said they would not urge their members to vote on the bill one way or the other. But they reversed course Tuesday evening, when the office of Minority Whip Steve Scalise (R-La.) released a notice recommending Republicans vote against the bill, saying it's too limited in scope and would duplicate other ongoing investigations into the attack.

Trump added his voice Tuesday night, calling the commission a "Democratic trap" and urging all Republicans to oppose the measure.

"Hopefully, Mitch McConnell and Kevin McCarthy are listening!" Trump said in a statement.

Katko has vowed to press on, characterizing his bill as "a solid, fair agreement" that marked "a dramatic improvement" over earlier versions he deemed more political.

But behind the calm public statements, colleagues say, Katko is furious that leaders left him dangling.

“Katko feels like he's been thrown under the bus,” one House Republican told The Hill on Tuesday. “I think he feels frustrated he was given a direction to go in and had the rug pulled out from under him.”

Democrats, according to Thompson, were equally stunned by the opposition from GOP leaders, who had endorsed the idea of creating a 9/11-style commission since the earliest days after the Jan. 6 attack. Thompson also disputed McCarthy's criticisms that Pelosi and Democrats had kept him in the dark through most of the drafting process.

"It was as big a shock to the Speaker and all of us that our minority leader had not — well, he even said he had not seen it. Which we have letters from him responding to it. So it's not like he hadn't seen it," Thompson said.

"It's unfortunate that the minority leader has, at the last moment, raised issues that basically we had gone past, and there was no issue on his part," he added. "But I guess that's politics."

A McCarthy spokesman did not respond Wednesday to a request for comment.

Despite the GOP opposition, the proposal is expected to sail through the House on Wednesday evening, with support from a small group of Republicans. Its fate is less certain in the Senate, where Minority Leader Mitch McConnell (R-Ky.) has said he'll vote against the measure. McConnell's opposition raises questions about whether the bill can win the 10 GOP votes to send it to President Biden's desk.