President Biden’s governing agenda is at risk of unraveling on Capitol Hill after a mounting series of delays, clashes and setbacks that have sapped momentum from an ambitious and intricate push to deliver on long-standing Democratic policy priorities.

An overhaul of the nation’s voting laws has been blocked by Republicans. An effort to strike a bipartisan deal on police reforms has lost all momentum. A sweeping immigration plan to provide a path to legal residency for millions of undocumented immigrants is now all but dead.

And centrist Democrats on Capitol Hill appear poised to pare down a planned $3.5 trillion economic package as their liberal counterparts in the House threaten in turn to kill the bipartisan infrastructure deal that passed the Senate last month.

Both chambers returned to session together Monday for the first time since July, as many Democrats acknowledged a sense of foreboding about the weeks ahead, with not only major aspects of their agenda on the line but also key deadlines looming to provide government funding and extend the federal debt limit.

The Senate parliamentarian serves as a check on rules and precedents. But recent partisan fights over policy have put the parliamentarian in the spotlight. (Blair Guild/The Washington Post)

“This is why we drew a very big circle at the outset, knowing that we might have our sails trimmed,” said Sen. Chris Murphy (D-Conn.). “This is all frustrating, because it’s clear that some really important things are going to be a lot harder.”

Murphy added that “there’s absolutely a pathway to get consensus on a set of things that fundamentally change people’s lives.”

But progress could depend on a new level of commitment and engagement from lawmakers and activists who have been more openly sparring with one another in recent weeks as obstacles have piled up.

The latest bad news came late Sunday from the Senate’s parliamentarian, who smacked down Democratic leaders’ attempts to insert sweeping immigration provisions into the party’s marquee “Build Back Better” legislation, ruling that the provisions did not qualify for special budget rules that could allow the bill to skirt a Republican filibuster.

To make the most of their razor-thin majority, Democrats are trying to pack as much as possible into a catchall bill that would expand health-care programs, grapple with climate change, extend the social safety net, lower prescription drug prices and pay for all of it by raising taxes on the wealthy. Unified Republican opposition to that sprawling agenda has eliminated any possibility of winning the 60 votes necessary to defeat a filibuster.

Senate rules allow for a simple majority to pass legislation under special budget procedures known as reconciliation, but those bills can include only provisions with a direct impact on federal revenue or spending. Proposals whose fiscal impact is “merely incidental” are not eligible for reconciliation — a test that the proposed immigration language did not meet, according to Parliamentarian Elizabeth MacDonough.

The proposals under consideration were vast, potentially granting legal residency to as many as 8 million immigrants, including the millions of “dreamers” — young undocumented immigrants who came to the United States as children and who have been the subject of years of negotiation and political wrangling, which have never culminated in a bipartisan agreement.

MacDonough, who was appointed under then-Majority Leader Harry M. Reid (D-Nev.) in 2012 and continued serving after Republicans took the majority in 2015, warned Democrats in her ruling that not every legislative proposal with a fiscal impact can be shoehorned into a reconciliation bill. The proposed immigration language, she wrote, represents a “tremendous and enduring policy change that dwarfs its budgetary impact.”

“The reasons that people risk their lives to come to this country — to escape religious and political persecution, famine, war, unspeakable violence and lack of opportunity in their home countries — cannot be measured in federal dollars,” she wrote in a memo to lawmakers.

That ruling vexed Senate Democrats, who had argued that several previous reconciliation bills included immigration provisions; Senate Majority Leader Charles E. Schumer (D-N.Y.) called the ruling “extremely disappointing.”

Republicans reveled in MacDonough’s decision.

“I’m glad Democrats failed in their effort to shove massive amnesty into the reckless taxing and spending spree they are assembling behind closed doors,” said Senate Minority Leader Mitch McConnell (R-Ky.). “This episode just shows how radical their far-left agenda has become.”

Furious liberal activists and immigrant advocates, many of whom called for MacDonough to be overruled or fired in light of the decision, are pushing Democrats not to give up.

Frank Sharry, the executive director of America’s Voice, a group promoting immigration changes, said the parliamentarian’s ruling amounted to a “middle finger” and “smacked of showboating.”

“If the parliamentarian is determined to get to no, then the question is, well, Democrats are the elected majority, and not a staff attorney — what do they plan to do?” he said.

Some of the hundreds who held a long-planned march Monday in Washington to call for a path to citizenship for millions of undocumented immigrants said they were devastated by the decision because they viewed Democrats’ combined control of the House, Senate and White House as a rare opportunity to secure gains after decades of marches, vigils and lobbying.

“It’s sad,” said Claudia Laínez, 43, who fled the aftermath of the civil war in El Salvador in 1994 and now is the mother of a U.S. citizen in Oakland, Calif. “It’s the only opportunity we have.”

Rep. Ayanna Pressley (D-Mass.) told the marchers — who are seeking a path to citizenship for holders of temporary protected status — that the Senate should disregard the parliamentarian and legalize millions of immigrants.

“This is about much more than process and procedures,” she said. “This is about real people.”

But key Democrats signaled Monday that they did not see procedural hardball as an option. Sen. Robert Menendez (D-N.J.), the most senior Latino member of the Senate, told reporters Monday that “I don’t think it’s necessarily constructive” to dismiss MacDonough, while Sen. Richard J. Durbin (D-Ill.), the No. 2 Senate leader, said Democrats simply did not have the votes to do it.

Several Democratic senators who disagreed with her ruling also said that any anger at MacDonough is misplaced, instead citing their colleagues’ unwillingness to change Senate rules to reform or eliminate the filibuster to pass legislation with a simple-majority vote.

“I think if we don’t face filibuster reform, and we don’t get a lot of these reforms done, there will be consequences in the election,” said Sen. Mazie Hirono (D-Hawaii).

The White House on Monday expressed hope that Democratic senators would find success in presenting fallback immigration proposals that could win the approval of the Senate parliamentarian and ultimately be included in the reconciliation package.

“There are a number of senators who have spoken to their intention of putting forward alternative proposals, because of their commitment, and our shared commitment, of course, to moving immigration reform forward and protecting dreamers and others,” said White House press secretary Jen Psaki.

MacDonough’s ruling marked the second time this year that she objected to including a major Democratic campaign priority in a reconciliation bill. Earlier, Democrats sought to add an increase in the federal minimum wage to a sweeping pandemic relief bill before being rebuffed.

She has not finished ruling on the pending legislation: Key decisions on climate and health-care provisions are still to come, and Democratic leaders said they are prepared to offer immigration alternatives for her review.

Beyond the minimum wage and immigration, talks have stalled over police reform, and lawmakers have shown little progress toward passing any new restrictions on guns, putting two other Democratic priorities at risk of falling by the wayside before voters head to the polls in 2022.

While Democrats have agreed among themselves on the need for federal voting rights legislation in the face of GOP efforts to tighten state restrictions, their proposal has not garnered any Republican support. A Senate vote on a revised bill could happen as soon as this week, but there is no expectation that the result will be any different from the one in June, when all 50 GOP senators opposed proceeding.

These developments have called into question how much of the program that Biden and his party ran on last year will actually be realized. Even amid signs that much of it could go unfulfilled, the White House sought to strike an optimistic note.

Asked Monday whether there is a recognition in the White House that some of the Democratic agenda is not going to happen, Psaki replied flatly, “No.”

One White House official, speaking on the condition of anonymity to describe private discussions, said the White House is seeking to tune out the noise and commentary about the agenda and stay focused on the work. Noting that others in party raised worries about the pandemic relief bill, which Biden signed into law, and the bipartisan infrastructure deal it reached in the Senate, the official said that not every development along the way is definitive.

White House officials in recent months have sought to underscore the steps they have taken unilaterally to address concerns in the party about voting rights and other issues, as Congress spins its wheels. But inside and outside the White House, there is acknowledgment that those steps are not nearly as robust as legislation would be.

Democratic pollster Fernand Amandi said there is an “existential urgency” to the party passing its agenda — and he warned that failing to do so could breed more of the anger at a broken government that fueled the rise of former president Donald Trump.

“This is more than a question about mere legislation — this is also a question about the very nature of America,” he said, adding, “The American voters did not give full control of the federal government and the legislative and executive branch so the Democrats could refer to the Senate parliamentarian.”

Paul Kane, Seung Min Kim and Marianna Sotomayor contributed to this report.