WASHINGTON (AP) — Republicans are poised to block legislation that would create a commission on the Jan. 6 insurrection, despite both a bipartisan effort to salvage the bill and a last-minute push by the mother of a Capitol Police officer who collapsed and died after the siege.

Senate Majority Leader Chuck Schumer has set up a Thursday procedural vote on the bill, challenging Republicans to support it after 35 of their GOP colleagues voted for it in the House. But it was unlikely that Democrats would be able to win the 10 Republican votes necessary to authorize the independent investigation, a remarkable turn of events just months after the worst attack on the Capitol in more than 200 years.

The bill as passed by the House would set up a bipartisan panel to investigate what happened when hundreds of former President Donald Trump’s supporters violently broke into the Capitol and interrupted the certification of President Joe Biden’s win.

On Wednesday, the mother of Capitol Police officer Brian Sicknick said she would meet with lawmakers ahead of the vote to try to convince them to act. Sicknick collapsed immediately after engaging with the rioters and died the next day.

“I suggest that all Congressmen and Senators who are against this Bill visit my son’s grave in Arlington National Cemetery and, while there, think about what their hurtful decisions will do to those officers who will be there for them going forward,” Gladys Sicknick said in a statement Wednesday. “Putting politics aside, wouldn’t they want to know the truth of what happened on January 6?”

A small number of Republicans, including Utah Sen. Mitt Romney and Alaska Sen. Lisa Murkowski, have said they expect to back the House-passed bill. Others, including Maine Sen. Susan Collins, have proposed small tweaks to the bill to try and attract more votes. But the effort had so far failed to yield additional support.

The talks come as Republicans have struggled over whether to support the bill — and with how to respond to the insurrection in general as many in their party have remained loyal to Trump. The former president told his supporters the morning of Jan. 6 to “fight like hell” to overturn his defeat and has repeatedly said the election was stolen, even though his claims have been refuted by courts and election officials across the country.

Most Republicans are expected to follow the lead of Senate Republican Leader Mitch McConnell, who put the issue in stark political terms Tuesday after meeting with his caucus. McConnell said that Democrats pushing the commission would like to litigate Trump’s actions and “continue to debate things that have been done in the past,” and that they should move to block it. The action would mark the first time Republicans have blocked significant legislation since Democrats claimed control of the Senate in January.

Trump has come out against it, calling the bill a Democratic “trap.” He urged his fellow Republicans to do the same.

McConnell voted in February to acquit Trump of inciting the insurrection after the House impeached him, but gave the speech immediately after that vote saying that the former president was “practically and morally responsible for provoking the events of that day.” He said last week that he was “open” to the House bill that would form a commission, but came out the next day against it.

Collins has said she is working with other senators to try to find a compromise, and West Virginia Democrat Joe Manchin has been part of that effort, according to two people familiar with the informal talks. The people spoke on condition of anonymity to discuss the private negotiations.

In a statement with Arizona Sen. Kyrsten Sinema, a fellow moderate Democrat, Manchin said that the attack was “horrific” and that the bipartisan commission is a “critical step.”

Four of the rioters died during the insurrection, including a woman who was shot and killed by police as she tried to break into the House chamber with lawmakers still inside.

Dozens of the officers defending the Capitol were brutally beaten by the rioters as they easily pushed past them and broke into the building, and Sicknick died the next day. Video shows two men spraying him and another officer with a chemical, but the Washington medical examiner said Sicknick suffered a stroke and died from natural causes. The men have been charged with assaulting the officers.

Collins’ amendment, released by her office Wednesday afternoon, would require the Democratic-appointed chair and the Republican vice chair to “jointly appoint” staff, changing House language that only required the chair to consult with the vice chair. It would also terminate the commission 30 days after a final report is issued, instead of 60 days, an effort to avoid their work spilling into the election year. Both the House version and Collins’ amendment would require the final report to be issued by Dec. 31, 2021.

“I want to see a commission, we need a commission, there are a lot of unanswered questions,” Collins said Wednesday. “I am working very hard to secure Republican votes.”

Still, most Republicans have held fast to their opposition. North Carolina Sen. Thom Tillis said that he had received text of the proposal from Collins, but that he was still “unlikely” to support the bill. He said he believes the year-end deadline is unrealistic.

Republicans have also pointed to a bipartisan Senate report that is expected to be released next month, saying it will be sufficient to fix security problems in the Capitol. The report by the Senate Rules Committee and the Senate Homeland and Governmental Affairs Committee is expected to focus on the mistakes made by law enforcement and the security command at the Capitol.

Senate Homeland Chair Gary Peters, D-Mich., said that his panel’s report will be important but that it was intended to identify ways to quickly secure the Capitol. There is more work to do, he said.

A commission “would be able to do a deeper dive into what led up to the attack on the Capitol, and be able to really spend the kind of time necessary to do a very thorough evaluation,” Peters said.

It’s so far unclear whether Schumer would be open to considering amendments to the legislation, if the Senate were to move ahead. He has repeatedly said that the procedural vote will show “where every member stands” on the insurrection and indicated that Democrats will use Republicans’ positions against them.

Congress “is not going to just sweep Jan. 6 under the rug,” Schumer said.

Associated Press writers Alan Fram, Colleen Long and Lisa Mascaro contributed to this report.