Pressure is building on President Biden, a longtime backer of traditional Washington rules, to do away with the filibuster and other procedures as Democrats press him to seize what could be a fleeting moment of power to enact his agenda.

Liberals have long pushed for sweeping changes such as expanding the Supreme Court, ending the electoral college and banning gerrymandering. But as Biden faces a critical stretch of his presidency, even moderate Democrats are urging more-immediate changes — particularly rewriting the filibuster so that at the very least fewer bills need 60 votes to pass the Senate.

Democrats increasingly worry that popular pieces of Biden’s agenda will hit a wall in the Senate, including his plans for climate change, immigration, gun control, voting rights and LGBT protections. Failing to enact them, they fear, could be a political disaster for Democrats.

Senate Majority Leader Charles E. Schumer (D-N.Y.) demurred when asked on March 2 if he would support ending the legislative filibuster. (The Washington Post)

Sen. Amy Klobuchar (D-Minn.), a centrist, said Wednesday she wants to “get rid of the filibuster,” her toughest comments to date on the matter. By Thursday, Sen. Tina Smith (D-Minn.) announced via social media that she, too, now wants to abolish the filibuster, because “the more I thought about it, the more I realized that the filibuster has long been the enemy of progress.”

Sen. Jon Tester (D-Mont.), also a moderate, told The Washington Post he could envision the Senate changing the filibuster if bills are floundering. “We’ve got to figure out whether leadership on both sides wants to have obstruction or if they want to come together and try to get some things done,” Tester said.

For the moment, Democrats do not have the votes to fully abolish the filibuster, which allows a senator to block a bill by refusing to yield the floor unless at least 60 senators overrule the speaker. Some Democrats, such as Sen. Joe Manchin III (W.Va.), oppose repeal. (“Never!” he shouted recently at a journalist who asked.) So advocates are looking for ways to limit the filibuster instead of ending it — and hoping Biden weighs in.

Sen. Jeff Merkley (D-Ore.) warned that if Democrats fail to pass a popular agenda because of arcane Senate rules, the party will suffer in the midterm elections. “It will be a Democratic Party Armageddon in 2022 if we sit here on our butts and say, ‘Oh, we’re sorry, we’re not as determined to get our agenda passed as Republicans were,’ ” said Merkley, who is spearheading an overhaul effort.

Republicans say such changes would create a free-for-all in the Senate and contend Democrats are threatening to toss the rules to gain an unearned political advantage.

“The same party that wants to change Senate rules when they lose a vote, pack the Supreme Court when they lose a case and throw out the electoral college every time they lose the White House now wants to forcibly rewrite 50 states’ election laws from Washington,” Senate Minority Leader Mitch McConnell (R-Ky.) said recently on the Senate floor, speaking in opposition to a Democratic election bill.

He added, “Millions of American voters elected 50 Republican senators and a whole lot of House Republicans to make sure that Democrats play by the rules, not rewrite the rules.”

As the presidential campaign unfolded and the depth of many Democrats’ dissatisfaction became clear, Biden softened his vociferous opposition to changing Washington’s rules. In July, he conceded that his approach to the filibuster would “depend on how obstreperous [Republicans] become.” After resisting proposals to expand the Supreme Court, he promised a commission to look into changes of the court’s structure.

Now that Biden is president, such middle positions could be harder to sustain. Biden faces a choice, some activists say, between ruling mostly via executive fiat — which permits modest accomplishments at best — and pulling down the structural obstacles.

For now, the White House is keeping its options on the table.

“One thing that is nonnegotiable is him delivering for the American people,” said Emmy Ruiz, the White House political director. “The number-one priority here is to get this agenda, this bold agenda, passed through Congress.”

A White House official, not authorized to the discuss the administration’s legislative approach and speaking on the condition of anonymity, said the “strategy is adjusting every single day,” reiterating Biden’s position that the filibuster is not sacrosanct, while the agenda is.

But with Democrats potentially losing their narrow House or Senate majorities in 2022 — a president’s party usually fares poorly in midterms — the Democrats’ window for change may be short-lived.

The vulnerability of Biden’s agenda became clear last week when a proposed minimum-wage increase ran into a procedural hurdle and was removed from his coronavirus relief package. Some Democrats, and many activists, saw that as a warning sign for the rest of Biden’s agenda.

And while Biden hopes to soon pass a $1.9 trillion coronavirus relief package, the second-largest stimulus bill ever to go through Congress, he may struggle to repeat that feat with measures that do not fit as easily into “reconciliation,” a maneuver allowing a bill to pass the Senate with a simple majority instead of 60 votes.

Biden will have just two more opportunities to use reconciliation before the midterms, and only budget-related bills qualify. It is the Senate parliamentarian who decides whether a bill fits under reconciliation, and while her advice can be ignored, Democrats have chosen to abide by her rulings.

“It’s not an ideal procedure to get things done, but politically this is the only palatable path right now to progress on key issues,” said Ben LaBolt, an aide in the Obama administration.

Some liberals in Congress sent a letter this week to Biden and Vice President Harris urging them to sidestep the parliamentarian’s decision on the minium wage, a move that has historical precedent.

“There is an institutional deference that maybe would have been fine in times past, but a defense of the status quo is inadequate to the challenges of our time,” said Rep. Ro Khanna (D-Calif.). “We have to follow the rule of law, but we don’t have to defer to traditions and norms.”

Without such changes, Senate rules force advocates into a tortuous process of making sometimes circuitous arguments for why their bills fit reconciliation.

Kerri Talbot, deputy director of the Immigration Hub, a pro-immigrant organization, said reconciliation may be the best hope for passing a citizenship measure for at least some of the 11 million undocumented immigrants in the United States. “We think we can qualify, due to the economic impact of providing a path to citizenship,” Talbot said. “Ultimately it’s a big boon for the economy, but in the short run there are some costs involved as well.”

Advocates and 100 lawmakers have asked Biden to consider legalizing at least 5 million undocumented immigrants via budget reconciliation. Three people with detailed understanding of Senate rules, however, said it is unlikely that a broad immigration plan would pass the parliamentarian’s muster.

Immigration activists are preparing for that eventuality. “We have to understand that the ruling of the Senate parliamentarian is not the end of the story,” said Carlos Rojas Rodriguez, who was among 150 supporters and ex-staffers of Sen. Bernie Sanders (I-Vt.) who wrote Sanders last week urging him to use reconciliation.

If the Senate parliamentarian disagrees, Rojas Rodriguez said, Democrats should overrule her.

Merkley said he is seeing steadily more receptiveness from his colleagues for ending the filibuster. In 2009 when he first joined the Senate, the push for change was “a very lonely journey,” he said, but now “there’s been a massive shift.”

Proponents are casting about for proposals that could win over the last few votes for change. For example, a current Democratic bill to overhaul elections is expected to attract no Republican support, prompting some Democrats to suggest an exception to the filibuster for civil rights and voting matters.

Biden’s climate agenda, a top priority for the party’s liberal wing and many young voters, also would probably struggle to attract 60 Senate votes, nor would it easily qualify for reconciliation. One lobbyist familiar with Biden’s plan said it would be unlikely to meet the reconciliation test without being substantially redrafted.

Some lawmakers believe that if a stack of popular bills passes the House but cannot get through the Senate, it would put critical pressure on Senate Democrats to consider revamping their system.

“The longer the Senate doesn’t function as it used to, pressure will keep building for changes that would allow overwhelmingly popular policies to move forward,” said Phil Schiliro, who headed legislative affairs in the Obama White House.

Senate Majority Leader Charles E. Schumer (D-N.Y.) has sidestepped questions about how Biden’s agenda could make it through the currently configured Senate.

“The bottom line is we’re going to come together as a caucus and figure out a way to get the bold action that the American people demand,” Schumer said recently. “We will put bills on the floor. We’re not going to be the legislative graveyard.”

Biden is uniquely situated to push for a major change to the Senate proceedings, some Democrats say, because of his credibility as a Senate institutionalist. He served in the chamber for more than three decades and frequently speaks of it with affection.

In his previous stint in the executive branch, Biden showed flexibility. As vice president he supported the Obama administration’s push to end the filibuster on most judicial nominations, lobbying his former colleagues to make the change, said Ed Pagano, a legislative-affairs aide in the White House at the time.

But Biden is also on the record defending Senate traditions such as the parliamentarian’s rulings, saying in a 2005 floor speech that heeding them had “been the practice for 218 years.”

As for killing the filibuster — that, he warned at the time, would be “a fundamental power grab by the majority party.”

Erica Werner, Alice Crites and Juliet Eilperin contributed to this report.