Earlier this summer, Al Sharpton traveled to Philadelphia to attend Joe Biden’s speech on voting rights. After delivering his remarks – a ringing call to arms against the march of restrictive election laws – the US president asked the veteran Black civil rights leader what he thought.

“I said it was almost like Lyndon Johnson the night he did his speech saying we shall overcome. But you missed one word, Mr President: filibuster,” Sharpton recalled in an interview with the Guardian.

“[Biden] said, ‘You’re not gonna get off of that, are you?’ And I said, ‘No, I’m not gonna get off of it until you do it.’”

The filibuster, a centuries-old parliamentary tool requiring 60 votes to advance most legislation in the Senate, now stands as the indomitable barrier to Democrats’ voting rights legislation. Biden has so far resisted calls from Sharpton and other prominent civil rights leaders to abolish the filibuster, even as he agrees with the characterization that it is a “relic of Jim Crow era”.

But Sharpton said that position was increasingly at odds with Biden’s avowed support for voting rights legislation, which advocates say is the last best hope of beating back a wave of restrictive voting laws in Republican-controlled states, inspired in part by Donald Trump’s baseless claims of voter fraud and that experts say disproportionately target communities of color.

At stake, the reverend said, was nothing less than the safeguard of American democracy.

“We’re going to have some issues that we’re going to disagree on, and we can respectfully disagree,” Sharpton said, referring to the president. “This is not one of them.”

Biden has forcefully and repeatedly called on the Senate to pass a pair of voting rights bills that Democrats have named as their highest priority: the For the People Act, a sprawling overhaul of federal elections, ethics and campaign finance laws, and the John Lewis Voting Rights Advancement Act, which would restore critical pieces of the 1965 Voting Rights Act after supreme court rulings gutted the law. Both measures passed the Democratic-controlled House, but are imperiled by Republican opposition in the evenly divided Senate, where – because of the filibuster – they will need support from at least 60 members to pass.

As the stalemate persists on Capitol Hill, Republican-led states are continuing to pass new measures restricting ballot access in what critics have described as the greatest contraction of voting rights since the Voting Rights Act was passed in 1965. This week, Governor Greg Abbott of Texas signed into law a bill that critics say will make it one of the hardest places in America to vote, particularly for people of color.

Filibuster reform remains unlikely, despite urgent calls and mounting outside pressure. Two centrist Democratic senators are opposed to eliminating the filibuster, Senators Joe Manchin of West Virginia and Kyrsten Sinema of Arizona.

In June, Sharpton was among a group of the nation’s top civil rights leaders that met with Manchin, after the West Virginia Democrat announced his opposition to the For the People Act.

During the virtual summit, Sharpton said they reminded Manchin that the filibuster was perhaps most famously used as a tool by segregationists and southern Democrats to block civil rights legislation. Manchin acknowledged its dark history, Sharpton said, but maintained that the filibuster had been more recently weaponized by both parties to thwart the will of the majority.

In the days that followed, the senator unexpectedly released a set of election reforms that he said he could support. But even if Democrats are able to unify behind a scaled-down version of the legislation, it would still face a steep climb to overcome a likely Republican filibuster.

As such, Sharpton believes there’s a possibility that Manchin could be persuaded to support a “carve-out” in the filibuster to allow voting rights bills to pass. He said the president’s unequivocal endorsement of filibuster reform would make it harder for Democratic holdouts such as Manchin to maintain their position. Even so, it is far from guaranteed that the president’s advocacy would move reluctant members of his party.

“We intend to keep pressuring the Biden administration on the carve-out around the filibuster,” Sharpton said, noting that Democrats and Republicans have created exemptions for confirming judicial nominees and supreme court justices. “If they could do carve-outs to confirm members of the supreme court for Donald Trump, they can do carve-outs to pass the John Lewis Voting Rights Advancement Act. This cannot happen without that.”

Sharpton and Biden have circled each other for decades. In the early 90s, Sharpton said he “marched on” Biden, then a US senator from Delaware and the author of crime legislation that helped pave the way for mass incarceration, which devastated Black communities.

More than 25 years later, Biden has evolved alongside the Democratic party on issues of criminal justice reform and racial equity. Sharpton said Biden’s embrace of racial justice issues – from police reform to public housing to voting rights – reflects how much the country has changed. But it also reflects, he said, the debt Biden owes to Black voters, who helped revive his stalled campaign during the Democratic primary and then helped deliver him the presidency in November.

“I’ve gone the whole way with him from fighting on the crime bill, where I still feel I was right, to where he is today,” Sharpton said. “And what I’ve said is that you, Mr President, said that Black America had your back and you’d have our back. Well, they’re sticking knives in our back. When do you come and have our back?

“There’s nothing more fundamental than voting,” he continued. “If we can’t depend on you here, when can we?”

Next week, Sharpton said he plans to visit Washington to pressure senators to advance the election bills languishing before them. In advance of the trip, he has requested a meeting with the senator minority leader, Mitch McConnell.

A meeting is probably unlikely, but Sharpton said he would appreciate the opportunity to discuss with the minority leader why voting rights legislation is not “unnecessary” as McConnell has said.

It is also part of a broader strategy by activists to show reluctant Democrats that Republicans are unwilling to compromise on voting rights and that filibuster reform is the only legislative path available to protect ballot access ahead of next year’s elections.

When lawmakers return to Washington next week, they face a tangled mess of urgent deadlines and legislative obstacles that will determine the fate of the president’s economic agenda, and whether a Democratic-controlled Congress can avert a government shutdown.

But Sharpton warned that a failure by the White House and Democratic leaders to prioritize votings rights would not only have far-reaching consequences for the electorate, it could also cost them their majorities in Congress, tenuous as they are.

“If the African American voters feel that there was not an aggressive fight and there’s low turnout, [Democrats] can easily lose the Senate and the House and Joe Biden will be an early lame duck,” Sharpton said. “They will be totally neutered if they don’t do something about this.”