WASHINGTON (AP) — Senate Republicans are ready to deploy the filibuster to block a commission on the Jan. 6 insurrection, shattering chances for a bipartisan probe of the deadly assault on the U.S. Capitol and reviving pressure to do away with the procedural tactic that critics say has lost its purpose.

The vote Thursday would be the first successful use of a filibuster in the Biden presidency to halt Senate legislative action. Most Republicans oppose the bill that would establish a commission to investigate the attack by Donald Trump supporters over the election.

“We have a mob overtake the Capitol, and we can’t get the Republicans to join us in making historic record of that event? That is sad,” said Sen. Dick Durbin of Illinois, the No. 2 Senate Democrat. “That tells you what’s wrong with the Senate and what’s wrong with the filibuster.”

The filibuster is a time-honored Senate tradition that requires a vote by 60 of the 100 senators to cut off debate and advance a bill. With the Senate evenly split 50-50, Democrats would need support of 10 Republicans to move to the commission bill, sparking fresh debate over whether the time has come to change the rules and lower the threshold to 51 votes to take up legislation.

The House already approved the measure with 35 Republican votes. But Trump has made it clear he opposes the formation of any panel to investigate the Jan. 6 siege, when a violent mob of his supporters broke into the Capitol in a failed effort to overturn Joe Biden's election. Democrats are warning that if Republicans are willing to use the filibuster to stop an arguably popular measure, it shows the limits of trying to broker compromises, particularly on bills related to election reforms or other aspects of the Democrats' agenda.

“There is no excuse for any Republican to vote against this commission,” said Sen. Joe Manchin, D-W.Va., while still making clear that he would not support efforts to do away with the filibuster.

"I’m not ready to destroy our government,” Manchin said. “I think we’ll come together. You have to have faith there’s 10 good people.”

Ahead of Thursday's vote, the mother of the late Capitol Police Officer Brian Sicknick, along with Sicknick's girlfriend Sandra Garza and two police officers who fought the protesters that day, met with several Republican senators to try to persuade them to act. Sicknick was among many officers protecting the building, some seen in videos in hand-to-hand combat with the mob. He collapsed immediately after engaging with the rioters and died the next day.

Video shows two men spraying Sicknick and another officer with a chemical, but the Washington medical examiner said he suffered a stroke and died from natural causes.

Outside of a meeting with Utah Sen. Mitt Romney, one of two senators who has signaled support for the House bill, Sicknick's mother said Sicknick was “doing his job and he got caught up in it, and it’s very sad.”

In a statement Wednesday, she was more blunt: “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.”

Garza and the officers also met with Maine Sen. Susan Collins, who has proposed minor changes to the bill to try to attract more GOP votes. Collins said afterward that they were “powerful advocates” for a commission, which she said is necessary to make sure “that such an attack on the ultimate symbol of our democracy never happens again.”

Collins' effort has so far failed to sway enough Republicans to support the legislation, and so far only Romney and Alaska Sen. Lisa Murkowski have said they will vote to move ahead with the bill.

"What we want is closure,” Murkowski said Wednesday.

Mostly, the GOP senators will follow Senate Republican leader Mitch McConnell, who has declared the bill a “purely political exercise,” since Senate committees are already looking into security shortfalls.

McConnell, who once said Trump was responsible for “provoking” the attack on the Capitol, now says of Democrats: “They'd like to continue to litigate the former president, into the future.”

The talks over potential changes to the legislation come as Republicans have struggled over how to respond to the insurrection 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. He has repeatedly said the election was stolen, even though his claims have been rejected by courts and election officials across the country.

Manchin, in a statement with Arizona Sen. Kyrsten Sinema, implored colleagues to act, calling the attack “horrific” and bipartisan commission a “critical step.”

The two centrist Democrats have been prominent holdouts against changing the Senate filibuster rules to allow 51 votes for passage of legislation.

Collins' amendment seeks to remedy some of the chief Republican complaints about the proposed commission. It would require the Democratic-appointed chair and the Republican vice chair of the panel 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.

Still, most Republicans have held fast to their opposition. Texas Sen. John Cornyn, who once supported the idea of the commission, said he now believes Democrats are trying to use it as a political tool.

“I don’t think this is the only way to get to the bottom of what happened,” Cornyn said.

Republicans have also pointed to a bipartisan Senate report that is expected next month, saying it will be sufficient to fix security problems in the Capitol.

Four protesters 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 injured. Hundreds of people have been arrested.

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