Senate Democrats failed again to pass sweeping new voting protections on Wednesday, in what may be the most brutal blow yet to efforts to strengthen protections for voters at a perilous moment for US democracy.

Just as they have done four other times in recent months, all 50 Republicans united in their opposition to the measure. They relied on the filibuster, a Senate rule that requires 60 votes to advance legislation to a final vote.

Despite heavy pressure from Joe Biden and fellow Democrats, two senators, Joe Manchin and Kyrsten Sinema, have dug in defending the measure, preventing Democrats from getting rid of it.

In a rebuke to Biden, Sinema gave a speech on the Senate floor last week making it clear she would not support changes to the filibuster. Manchin has also consistently made his support clear. “I will not vote to eliminate or weaken the filibuster. The filibuster plays an important role in protecting our democracy from the transitory passions of the majority and respecting the input of the minority in the Senate,” he said in a speech on Wednesday.

Their opposition set up a showdown as the ultimately doomed bill was taken up for discussion on Wednesday. Late in the evening, Republicans used the filibuster to vote to end debate on the bill, effectively blocking it from advancing. Immediately afterwards, Democrats moved to hold a vote to try and change the filibuster rules anyway. The effort failed 52-48, with Manchin and Sinema voting with all 50 Republicans to preserve the filibuster. Sinema loudly said “aye” when it was her turn to vote in favor of preserving the filibuster changes.

“I am profoundly disappointed that the United States Senate has failed to stand up for our democracy. I am disappointed — but I am not deterred,” Biden said in a statement.

“Our Administration will continue to fight to pass federal legislation to secure the right to vote. We will not stop fighting against the anti-voter legislation that Republican legislatures continue to push at the state level—and to champion and support state and local elected officials who work to enact pro-voter legislation,” Kamala Harris said in a separate statement.

“Isn’t protecting voting rights, the most fundamental wellspring of this democracy, more important than a rule?” Chuck Schumer, the Senate majority leader, said just before the vote on the filibuster change.

Senator Tim Kaine of Virginia said Democrats were seeking to restore a “talking filibuster”, where senators have to hold the floor of the US senate to prevent a vote on legislation.

“We’re going to take up a rules reform proposal that will not blow up the senate,” he said on the Senate floor Wednesday evening. “It switches the secret filibuster into a public filibuster. It makes both parties work on the floor to get the kind of extended public debate we joined together to seek.”

Senator Angus King of Maine, who once defended the filibuster, said the process that was in place was a “second cousin once removed of the filibuster”.

“I’d venture to say if we had the rules we have today, we wouldn’t have the Voting Rights Act and the Civil Rights Act,” he said.

Mitch McConnell, the Republican leader, described Wednesday as “in all likelihood, the most important day in the history of the Senate.” He said the Democratic proposal was just “smoke and mirrors”, and accused Democrats of undertaking a plot to “to break the Senate”.

The voting rights measure has failed before, but Wednesday marks the first time they have taken a formal vote on changing the filibuster. Its likely failure marks a profound setback for Biden’s presidential agenda. The president spent an enormous amount of political capital in recent weeks pressuring Manchin and Sinema to support rule changes to the filibuster, giving a speech in Atlanta and traveling to Capitol Hill to try to get support.

In stirring remarks just before the vote on the voting rights bill, Raphael Warnock, a Democrat from Georgia, said senators could not praise the legacy of Dr Martin Luther King, Jr while voting against voting rights. “You cannot remember MLK and dismember his legacy at the same time,” Warnock said. “I will not sit quietly while some make Dr King a victim of identity theft.”

“Those of us who are students of Dr King, I know I have, often wonder ‘what would I have done if I was alive during the civil rights movement?’ I know that we all would like to think we had a fraction, just a small fraction of the courage it took for John Lewis to cross that Edmund Pettus Bridge,” he said. “Well, for those of us who serve in the United States Senate in this moment, in this moral moment, we do not have to wonder … we don’t have to wonder what we would have done. I submit that what we would have done back then we are doing right now. History is watching us.”

The bill that failed on Wednesday, Freedom to Vote: John R Lewis Act, combined two major voting rights bills into a single mega bill.

It would have set a national baseline for election access, guaranteeing 15 days of early voting as well as online voter registration. It protected local election officials from harassment and partisan interference in their jobs and curbed gerrymandering, the severe distortion of partisan district lines. It also restored a key piece of the 1965 Voting Rights Act that required places with a history of voting discrimination to get their changes approved by either the justice department or a federal court in Washington before they go into effect.

The bill’s failure comes as states across the US have waged an aggressive effort to restrict voting access after the 2020 election, which saw record turnout. In total, 19 states have passed 34 bills that restrict voting access, making it harder to request and return a mail-in ballot, among other measures, even though there was no evidence of fraud, either in mail-in voting or otherwise, in 2020.

Many of those efforts are obviously aimed at Black and other minority voters who helped Democrats win in 2020, activists say. As state legislatures reconvene, Republican lawmakers are proposing even more new restrictions.

At the same time, Republicans in state legislatures are redrawing electoral districts at the state legislative and congressional level to virtually guarantee their re-election for the next decade. Seeing Democratic gains in traditionally Republican districts, Republicans have redrawn the lines to simply make many districts uncompetitive for the next decade, according to the Brennan Center for Justice.

It’s a strategy that has blunted the growing power of Black, Hispanic and Asian voters in places like the suburbs, which are rapidly diversifying. In North Carolina, for example, Republicans lowered the Black voting age population of a district long represented by GK Butterfield, a former judge who is Black. It will be harder for Black voters in that district to elect their candidate of choice and Butterfield has since announced he is retiring from Congress.

There is also growing concern about what experts call election subversion – efforts to inject more partisanship into election administration and counting votes.

Republicans are passing laws that give them more partisan control over key administrative roles and Trump allies who have embraced the myth of a stolen election are running for secretary of state in places such as Georgia, Michigan, Arizona and Nevada – a perch from which they could exert enormous unilateral control over election rules.

Civil rights groups have waged an aggressive campaign, privately and publicly, trying to get Manchin and Sinema to support a filibuster change. “Let’s be honest, every voter suppression bill passed in the 19 states across the country has been passed by Republicans alone. If one party can dismantle our democracy on its own, the other party should muster the courage to safeguard it,” Derrick Johnson, the president and CEO of the NAACP, wrote in a letter to senators.