Six months into his role as majority leader, Senator Chuck Schumer is still trying to get Democrats’ legislative agenda into full gear. But the Senate Democratic caucus has a built-in brake system. Its name is Joe Manchin III.

And on Sunday, Mr. Manchin made it clearer than ever that he was uninterested in helping the Democrats get to cruising speed.

In an opinion essay in his hometown newspaper, The Charleston Gazette-Mail, Mr. Manchin declared that he would vote against the Democrats’ sweeping voting-rights bill called the For the People Act, and he promised in no uncertain terms to oppose eliminating or reforming the filibuster.

The announcement, though not altogether a surprise, dealt a crushing blow to the hopes of most Democratic lawmakers, who had seen the upcoming summer session as their big opportunity to advance the party’s ambitious goals — on voting rights and beyond. (After the chamber adjourns for the August recess, much attention will turn toward next year’s midterm elections.)

It also serves to clarify where the goal posts may lie on a variety of issues, as Mr. Manchin stands firmly against allowing his party to legislate freely without buy-in from Republicans.

“I will not vote to weaken or eliminate the filibuster,” he wrote. “For as long as I have the privilege of being your U.S. senator, I will fight to represent the people of West Virginia, to seek bipartisan compromise no matter how difficult and to develop the political bonds that end divisions and help unite the country we love.”

After Mr. Manchin published his essay, Mr. Schumer tried to sound an optimistic note by mentioning two areas where legislative proposals might soon get a vote in the Senate: gun control and L.G.B.T.Q. rights. But there is little certainty about whether those bills could pass at this point.

The same is true of some of the biggest-ticket Democratic priorities in the weeks and months ahead. Here’s a look at where things stand on four of President Biden, Mr. Schumer and Speaker Nancy Pelosi’s main goals, in light of Mr. Manchin’s ultimatum of bipartisanship.

Voting rights

The most immediate (likely) casualty is the For the People Act. It is known as S. 1 in the Senate and H.R. 1 in the House, reflecting its pride of place among Democratic leaders, who consider it to be an existential necessity as Republicans in state legislatures across the country pass laws that restrict voting access and may make it easier for state lawmakers to challenge or invalidate election results they find unfavorable.

In his essay, Mr. Manchin aimed most of his darts at fellow Democrats, branding the For the People Act as a partisan power grab. This is the main argument against the legislation used by Republicans to give cover to the state-level bills, including Senator Mitch McConnell, who has said S. 1 is “designed to advantage one side and disadvantage the other.”

Mr. Manchin wrote, “Today’s debate about how to best protect our right to vote and to hold elections, however, is not about finding common ground, but seeking partisan advantage.”

He went on to explain himself more fully: “I have always said, ‘If I can’t go home and explain it, I can’t vote for it.’ And I cannot explain strictly partisan election reform or blowing up the Senate rules to expedite one party’s agenda.”

Representative Jamaal Bowman, a liberal Democrat from New York, accused Mr. Manchin of doing Mr. McConnell’s job for him. “Joe Manchin has become the new Mitch McConnell,” Mr. Bowman said during an appearance on CNN.

“Mitch McConnell, during Obama’s presidency, said he would do everything in his power to stop Obama — he’s also repeated that now during the Biden presidency,” he added. “Now Joe Manchin is doing everything in his power to stop democracy and to stop our work for the people, the work that the people sent us here to do.”

Mr. Manchin has said that while he’s opposed to S. 1, he would support a version of the John Lewis Voting Rights Act, which seeks to restore provisions of the 1965 Voting Rights Act invalidated by the Supreme Court in 2013, but it would do little to turn back the laws already passed by Republican legislatures.

Infrastructure and climate

Voting rights is far from the only issue on which Mr. Manchin has said he wants bipartisan agreement. He’s also brandished the demand for bipartisanship against Mr. Biden’s major domestic-policy priority: investment in infrastructure.

Mr. Manchin has said that he strongly supports federal legislation to support jobs and strengthen the country’s infrastructure, though he objects to elements of Mr. Biden’s $2 trillion American Jobs Plan, including raising the corporate tax rate to 28 percent.

The White House has been in talks with the other senator from West Virginia — Shelley Moore Capito, a Republican — seeking a compromise proposal. There have been stops and starts in recent days, but Ms. Capito and Mr. Biden are continuing their meetings this week, and Mr. Manchin has described himself as encouraged by the seemingly good-faith negotiations. “I still have all the confidence in the world,” he said on Fox News yesterday. “My goodness, the president has gone from $2.25 trillion down to $1 trillion. The Republicans have come up quite a bit from where they started.”

He and seven other senators — including the Republicans Mitt Romney and Rob Portman, and Kyrsten Sinema, a Democrat — are at work separately on a compromise bill that could provide a fallback option in case Ms. Capito’s talks fail.

It would also be a way for Mr. Manchin to keep the pressure on Mr. Biden to negotiate further with Republicans, rather than passing an ambitious bill with only Democratic support.

An infrastructure bill could become law through the budgetary reconciliation process, meaning that only Democratic votes would be needed. But Mr. Manchin would have to agree to that, and so far he hasn’t.

Democratic leaders have also floated the idea of passing a smaller-scale, bipartisan infrastructure bill and then following that with a larger, Democratic-only bill that expands upon it.

Competing with China

The Senate’s most immediate legislative priority is also the rare bill on the docket that’s now expected to gather bipartisan support. The Endless Frontier Act, which would invest almost a quarter of a trillion dollars in U.S. industries, is being framed as an attempt to directly beef up U.S. competitiveness with Chinese state-backed manufacturers.

The Senate could vote on the bill as early as tomorrow. On the campaign trail last year, Democrats and Republicans often sought to outdo each other on anti-China language, and senators in both parties have expressed support for legislation aimed at taking on a country that’s seen as America’s fastest-rising economic rival.

The legislation would invest $52 billion in subsidizing semiconductor manufacturers in the U.S., and an additional $195 billion would go toward scientific research and development.

Police reform

After more than a year of renewed protests against racism in law enforcement, senators in both parties have indicated that they would back a police reform bill. But the details remain sketchy, and finding at least 10 Republican votes would be an uphill climb, considering the staunch opposition of police unions across the country to most of the provisions being discussed.

In March, the House passed the George Floyd Justice in Policing Act, which would ban certain practices like chokeholds and take steps to increase accountability for officers charged with wrongdoing.

That bill garnered virtually no Republican support in the House, and Senator Tim Scott — the only Black Republican in the Senate and the G.O.P.’s point person on police reform — has committed to working with Democrats toward a compromise. It is unlikely that Republicans would support some of the provisions most sought by activists, particularly a rollback of qualified immunity, the Supreme Court doctrine that prevents most kinds of civil suits against accused officers.

Mr. Manchin, for his part, has been tight-lipped on his support for the bill. Activists have gone as far as reading his name aloud at protests, encouraging activists to phone Mr. Manchin’s office and urge him to support the legislation.