The bipartisan push to launch an independent and nonpartisan investigation of the Jan. 6 riot at the U.S. Capitol suffered a fatal blow Friday, after nearly all Senate Republicans banded together in opposition.

The 54-to-35 outcome, six votes shy of the 60 needed to circumvent a procedural filibuster, followed hours of overnight chaos as lawmakers haggled over unrelated legislation. The vote stood as a blunt rejection by Republicans of an emotional last-minute appeal from the family of a Capitol Police officer who died after responding to the insurrection, as well as an 11th-hour bid by Sen. Susan Collins (R-Maine) to save the measure by introducing changes intended to address her party’s principal objections.

In its wake, many senators who had supported the commission were openly angry, as even the Democrats’ most moderate senator blamed Senate Minority Leader Mitch McConnell (R-Ky.) for killing a bill in order to score political points, instead of doing what was right.

Sen. Joe Manchin III (D-W.Va.) told reporters that there were “an awful lot of other Republicans that would have supported” the commission “if it hadn’t been for his intervention,” guessing that but for McConnell’s whipping, “13 or 14” GOP senators might have voted for the bill.

In the past two weeks, only a handful of Republican senators expressed positive sentiments about a commission. On Friday, six of them — Sens. Bill Cassidy (La.), Lisa Murkowski (Alaska), Rob Portman (Ohio), Mitt Romney (Utah), Ben Sasse (Neb.) and Collins — joined all voting Democrats to back the commission. All except Portman voted earlier this year to convict Trump on impeachment charges for inciting an insurrection.

Another 11 — nine Republicans and two Democrats — did not participate. Though the vote was held on the last day before senators were scheduled to take a week-long break, it was striking that so many missed such a high-profile vote — especially because some had voiced positive opinions about the proposed commission in recent days.

Both Democrats who missed the vote, Sens. Patty Murray (Wash.) and Kyrsten Sinema (Ariz.), along with Republican Sen. Patrick J. Toomey (Pa.), would have cast yes votes had they been present, bringing the commission legislation within three of the 60 it needed to proceed. Murray needed to fly home for a personal matter, she said via Twitter. Sinema’s office did not immediately respond to a request for comment. Toomey had a family commitment, his spokesman said.

At least one other of those nine Republicans — Sen. Mike Rounds (S.D.) — issued positive statements about a commission last week, only to walk them back on the eve of the vote.

“I still would like to see a commission go through, just for history’s sake, I’d like to see it — but I think we’re going to have to wait until after the criminal prosecutions are completed,” Rounds told reporters Thursday, arguing that the commission would have difficulty accessing witnesses and information tied up in court cases. “Practically speaking, we just can’t do it at this point.”

The commission legislation was a product of cross-party negotiations among leaders of the House Homeland Security Committee, and it had galvanized significant support among Republicans in the lower chamber. Last week, 35 GOP House members joined all voting House Democrats to back the creation of a Jan. 6 commission, to be modeled after a similar independent panel formed in the aftermath of 9/11 and charged with producing an objective account of what fueled the day’s violence.

But in the Senate, Republican sentiment soured after McConnell dismissed the commission as needlessly duplicative of congressional probes — and as a Trojan horse that would help Democrats in next year’s midterm elections.

“I do not believe the additional, extraneous ‘commission’ that Democratic leaders want would uncover crucial new facts or promote healing,” McConnell said Thursday, arguing that the Justice Department and Senate committees were already handling that substantive work. “. . . I’ll continue to urge my colleagues to oppose this extraneous layer when the time comes for the Senate to vote.”

Former president Donald Trump, whose most zealous supporters carried out the attack, has cast a long shadow over the GOP as lawmakers have wrestled with the proposal to establish a 10-person panel of nongovernment experts charged with finding answers — and accountability. The proposal called for five members, including the chair, to be appointed by Democrats and five, including the vice chair, to be appointed by Republicans. The commission would have had the power to issue subpoenas on a bipartisan basis, which some Democrats warned — and many Republicans worried — could have been used to force the former president and his allies in Congress to testify under oath.

Over the past week, GOP senators voiced concern that even if the commissioners’ ranks were bipartisan, the panel’s staffing might not be. They also argued that if the panel did not produce a final report before the end of the year, Republican lawmakers would have to spend much of the 2022 campaign season responding to its revelations about Trump’s past ills and trying to sidestep his outbursts, when their aim is to make the next election cycle a referendum on President Biden and the Democrats who control Congress.

Collins tried to address both points with an amendment that would have required the commission’s chair and vice chair to make hires together and shortened the time it would have to wind down its work after the Dec. 31 deadline to issue a final report. But while her proposed changes generated a flurry of last-minute activity around the bill, they never came to a vote on the floor.

As the vote began Friday, Collins had a visibly angry reaction, confronting Senate Majority Leader Charles E. Schumer (D-N.Y.) and accusing him of undermining the vote by not declaring ahead of time that he and the Democrats would back her changes if the measure to take up the bill advanced.

A spokeswoman for Portman said the Ohio Republican had sought and secured a personal assurance from Schumer on Friday that Collins’s amendment would be voted on, before throwing his support behind the commission. Toomey’s spokesman said the senator also would have voted for the commission “with the expectation” that the Senate would have then voted on Collins’s changes.

Trump entered the fray last week, warning that the commission was a “Democrat trap” and excoriating the “35 wayward Republicans” who supported the proposal in the House.

“Sometimes there are consequences to being ineffective and weak,” he said in a statement, issuing a personal challenge to McConnell and House Minority Leader Kevin McCarthy (R-Calif.) to heed his warnings.

The GOP’s reluctance to hold Trump accountable for allegedly inciting the riot began just days after the violent incident, when Democrats responded by impeaching him for a second time, an effort in which only 10 House Republicans joined. A month later, a majority of Senate Republicans voted to acquit him based on a widely challenged argument that the Constitution does not permit the conviction of former presidents.

The GOP’s votes stood in sharp contrast to its prevailing rhetoric at the time, which was sharply critical of Trump. ­McConnell, immediately after voting to acquit the former president, accused him of inciting the insurrection. Yet in recent weeks such criticism largely fell silent as Republicans muzzled anti-Trump sentiment within their ranks, even ousting the third-ranking House Republican, Rep. Liz Cheney (Wyo.), from her leadership position over her campaign to hold Trump accountable for the riot.

Instead, the party targeted Democrats, suggesting that they harbored ulterior political motives in rallying the votes for an outside investigative commission.

Still, some senators have warned that if the GOP fears the politicization of an investigation, an independent commission is the least of all possible evils. This week, Cassidy, one of just seven Senate Republicans to vote for Trump’s impeachment conviction earlier this year, warned that if a commission did not come to fruition, a House select committee would probably take its place — pitching Jan. 6-related discussions into a forum that would be even more politically disadvantageous for Republicans.

But Cassidy’s sentiments proved to be unique, as most other GOP senators banded around the argument that a commission would be “extraneous,” as McConnell put it, given other investigations of the same events.

At least six committees in the House and Senate have held competing hearings investigating the insurrection, leading to scattered revelations — and thus far no conclusions. Instead, most of those hearings have been plagued by partisan sniping, while a highly anticipated interim report from the Senate Rules and Homeland Security and Governmental Affairs committees, which are jointly investigating the riot, has been delayed by several weeks amid struggles to reach bipartisan agreement about their findings.

In a statement Friday condemning McConnell and other Republicans who refused to support the commission, House Speaker Nancy Pelosi (D-Calif.) vowed, without offering specifics, that her party would “proceed to find the truth.” The GOP, she said, “clearly put their election concerns above the security of the Congress and country.”

On Thursday, family and friends of Capitol Police officer Brian D. Sicknick attempted to make a personal moral appeal to GOP senators, in the hopes of shaking up the political stalemate and turning votes in favor of the commission. Sicknick suffered two strokes and died of natural causes a day after he confronted rioters at the insurrection, the District’s chief medical examiner ruled last month. But after meeting with 15 senators, Sandra Garza, the late officer’s partner, emerged deflated.

“Why would they not want to get to the bottom of such horrific violence?” she said to reporters. “It just boggles my mind.”

The family met with Republican senators who were committed to opposing the commission and those who had already declared their intention to support it, as well as a group on the fence. Late Thursday night, Murkowski, one of the Republicans who promised to back the commission ahead of the vote, recounted telling Sicknick’s mother, Gladys Sicknick, that she was “heartsick that you feel you need to advocate to members of Congress that we stand up and say, ‘The truth is hard, but the truth is necessary.’ ”

Mike DeBonis and Felicia Sonmez contributed to this report.