Democrats grappled Monday with the seeming decimation of their sweeping legislative ambitions at the hands of one of their own lawmakers, Sen. Joe Manchin III of West Virginia, who quashed prospects for the party’s marquee voting rights bill over the weekend and cast a pall over other planks of President Biden’s agenda.

Manchin’s blow came in an op-ed published Sunday by the Charleston Gazette-Mail, in which he declared his opposition to the For the People Act — a sweeping measure meant in part to override new voting restrictions passed by GOP legislatures — and reiterated his vow never to repeal or modify the Senate’s 60-vote supermajority rule known as the filibuster.

Together, Manchin’s statement was a bracing reminder of the precarious Democratic majority, guaranteed only by Vice President Harris’s vote in a 50-50 Senate, where their entire governing agenda rides on the whims of any one senator — including one, in Manchin, who revels in his image as a party-bucking maverick who has made bipartisanship his political calling card.

Inside the Senate on Monday, Manchin’s Democratic colleagues reacted with surprise and disappointment — largely keeping any personal frustrations in check, given the mutual acknowledgment that Manchin’s vote is essential to success on infrastructure, confirming Biden’s nominees and several other priorities.

Sen. Chris Murphy (D-Conn.) said Democrats urgently need to find a way to move past their internal divides and make progress. “If we’re going to shut down the Senate and not do anything big between now and next election, we might as well hand the election to the Republicans,” he said Monday. “If we’re not going to act in big and bold ways to protect our democracy and help our economy, then we’re in power for another year and a half.”

Here’s what you need to know about the procedure’s complicated history meant to delay, delay, delay. (Drea Cornejo/The Washington Post)

In the immediate term, Manchin’s stance has given new urgency to bipartisan negotiations on infrastructure spending — Biden’s top domestic priority — which have appeared to falter as the two parties confront a spending gap of hundreds of billions of dollars. But Manchin, in interviews with CNN and NBC last week, said he was not ready to abandon the talks and support using expedited budget procedures to pass legislation on a partisan basis.

Should Democrats and Republican fail to broker a deal, the White House will need every Democratic senator to rally behind the infrastructure bill on a party-line vote, making Manchin a pivotal figure capable of making or breaking a centerpiece of the Biden agenda.

“The president considers Senator Manchin a friend. He knows that they may disagree on some issues,” said White House press secretary Jen Psaki, who argued that Manchin’s opposition to Democratic priorities will not doom Biden’s agenda. “He’s going to continue to work with him, reach out to him, engage with him directly.”

Her comments reflected the careful line the White House is walking as Manchin’s vote appears increasingly crucial to the giant infrastructure package Biden has prioritized.

But the special budgetary procedure Democrats could use to pass an infrastructure bill with a simple majority is not an option for voting legislation — or for gun-control bills, gender pay equity legislation, a minimum-wage hike, union organizing protections and other major pieces of the Biden agenda.

On one hand, Manchin’s weekend statement was not a complete surprise. He has long defended the filibuster as a bulwark of bipartisanship in the Senate and declared himself opposed to its elimination earlier this year, even as more and more of his colleagues have diagnosed it as cause of the chamber’s dysfunction. And he shared misgivings about the voting bill as far back as March, expressing concerns about the lack of Republican support for the bill without challenging any of its specific provisions.

But his decision to openly oppose the bill dashed the hopes of Democratic senators who believed he could be cajoled into joining the rest of his party on a matter some lawmakers and activists have cast as an existential matter for American democracy — if nothing else, to show that Democrats were united against Republican attempts to narrow voting rights.

Senate Majority Leader Charles E. Schumer (D-N.Y.) did not mention Manchin in his floor remarks Monday and indicated that he will move forward with his plan to bring the voting bill to the Senate floor later this month for a test vote. Some Democratic aides said they still believed Manchin could be persuaded to vote to at least begin debating and amending the bill, though even that step would be subject to a GOP filibuster.

Manchin’s logic for opposing to bill — that it simply is not sufficiently bipartisan — is alien to most of his colleagues, who have watched Republican legislatures in several states pass voting restrictions on straight party lines, over the fierce objections of Democrats in those states.

He had little comment Monday as he arrived at the Senate to vote, rushing quickly past reporters and brushing off the criticism he has gotten from fellow Democrats. “They’re all my friends,” he said.

Although Manchin was tight-lipped about his strategy on Monday, some Democrats said they interpreted his op-ed as a testament to the dueling pressures he faces to support Biden’s proposals in Washington and demonstrate some independence from his party back home. Bucking his party on voting legislation could give him more flexibility to eventually fall in line with them on infrastructure, these Democrats said.Senate Minority Leader Mitch McConnell (R-Ky.) has rallied his members against the For the People Act, calling it an unwarranted federal intrusion on the right of states to determine their own voting laws and a “brazen political power grab” — as he put it Monday — by Democrats hoping to aggrandize their own power. No Republicans have joined the bill.

McConnell made a veiled reference to Manchin’s opposition to the voting bill in floor remarks Monday.

“The question isn’t even whether it could earn bipartisan support,” he said. “The question is how wide the bipartisan opposition will be.”

In a demonstration of how the filibuster threatens to block even minor pieces of the Democratic agenda, Senate Republicans are also expected to block consideration of a bill this week that aims to narrow the gender pay gap.

Democrats have tried to cast the stakes for Biden’s agenda as higher than routine legislative disagreements when arguing why Manchin should drop his insistence for bipartisan compromises that don’t seem likely.

“History will haunt members of the United States Senate who allow the franchise to be radically diminished and corruption to be even more prevalent,” warned Sen. Richard Blumenthal (D-Conn.) on Monday. “People will be judged on their votes, not their party.”

Among those taking a gentler tone Monday was Sen. Raphael G. Warnock (D-Ga.), a freshman who has sought to rally his colleagues to combat the GOP-passed voting laws in his own state and others in increasingly urgent terms. Warnock said he had spoken with Manchin on Monday and remained “hopeful” he might ultimately support some type of voting legislation.

“I think that Joe Manchin understands that this is a defining moment in American history and that our children are going to judge us, our grandchildren are going to judge us, based on what we do right now to preserve our democracy,” he said.

Senate Majority Whip Richard J. Durbin (D-Ill.), the party’s top vote-counter, said he simply wanted to have a conversation with Manchin about his views on the voting bill.

“I just want to really understand what’s at the heart of it,” he said.

Durbin said a more aggressive approach at pressuring Manchin, who was reelected to a six-year term in 2018 in a state that twice voted overwhelmingly for former president Donald Trump, would not produce a different result.

The already tense talks among Senate Democrats about voting rights are expected to continue at a weekly caucus meeting on Tuesday, said one Democrat planning to attend, who was surprised by Manchin’s op-ed and spoke on the condition of anonymity to discuss a private gathering.

Among a group of House liberals who had much stronger words for Manchin on Monday was Rep. Mondaire Jones (D-N.Y.), who criticized Manchin’s op-ed in an interview as “intellectually unserious and cynical” and accused him of “clinging to a bygone era” where bipartisanship on voting rights matters were routine.

Jones called on Biden to intervene and use whatever means he had available to persuade Manchin to back legislation and change the Senate rules to pass it.

“The president of the United States — the leader of the free world and the most powerful person on the planet — must use the resources available to him to negotiate a deal with a member of his own party to preserve the union and save our republic and American democracy,” Jones said. “As yet, this president has not risen to the occasion.”

Manchin on Tuesday is set to meet with a host of civil rights leaders, including the Rev. Al Sharpton and the heads of the NAACP, the National Urban League, the National Council of Negro Women, the Lawyers’ Committee for Civil Rights Under Law, and the Leadership Conference on Civil and Human Rights.

Meanwhile, a prominent liberal activist said he would lead a “Moral March on Manchin” next week in the senator’s home state and conduct a “nonviolent direct action” targeting Manchin in Washington.

The Rev. Dr. William J. Barber II, leader of the Poor People’s Campaign, tweeted Monday that Manchin had “abandoned” the poor residents of West Virginia and called his political positions “wrong, constitutionally inconsistent, historically inaccurate, morally indefensible, economically insane, and politically unacceptable.”

The United Mine Workers of America, an influential group in Manchin’s state, reiterated its support for voting rights legislation on Monday, citing restrictive laws being passed by Republican legislatures in some states. “It is wrong for these states to attack the basic rights of citizens to participate in our democracy,” UMWA International President Cecil E. Roberts said in a statement. “Congress should be doing everything possible to not just maintain, but expand voting access and create freer and fairer elections. If only one party is interested in doing that, then so be it.”

Psaki said that Biden continues to favor moving forward with the John Lewis Voting Rights Act as well as the For the People Act. She declined to say whether Manchin gave the White House a heads-up on his op-ed in advance of its publication.

Asked whether Biden and Manchin have an agreement for the West Virginia Democrat to support using reconciliation to support the infrastructure plan if the bipartisan negotiations fail, Psaki declined to say. “I’m certainly not going to speak on behalf of where Senator Manchin is,” she said. But Psaki was quick to point out what she called “positive signs” about his position on some aspects of the plan.

White House officials have sought to underscore the steps they have taken through the executive branch to protect and expand voting rights. But those measures have not stopped Republican state governments from imposing new restrictions in recent months. Democrats have increasingly grown worried about the battle over voting rights, and one White House official said the issue could have global implications.

“I would say the basic notion of democratic reform and voting rights in the United States is a national security issue,” White House national security adviser Jake Sullivan told reporters. “We are in a competition of models with autocracies and we are trying to show the world that American democracy and democracy writ large can work.”