House Democrats are preparing to take the first steps Monday toward adopting a roughly $3.5 trillion spending plan that would enable sweeping changes to the nation’s health care, education and tax laws, but new rifts among party lawmakers threaten to stall the package’s swift advance.

The budget blueprint encompasses many of Democrats’ most cherished policy promises from the 2020 campaign, including pledges to expand Medicare, rethink immigration, and spend new sums to combat climate change. Its adoption this week would inch Congress closer to delivering on President Biden’s broader economic agenda.

But the fate of that vote appears in doubt, as House Speaker Nancy Pelosi (D-Calif.) continues to grapple with persistent political divides among her own fractious caucus. Despite wide-ranging support for some of the new spending, the party’s liberal and centrist wings remain at odds over how exactly to proceed, raising the potential for defections that Democrats simply cannot afford in a chamber where they hold only a slim advantage.

The tensions have played out over what should have been a routine process to bring the budget to a final vote. Nine centrists, led by Rep. Josh Gottheimer (D-N.J.), have signaled they could vote against the proposal unless Pelosi first permits a vote on a bipartisan, roughly $1.2 trillion bill to improve the country’s infrastructure. The Senate adopted both packages before its recess this month.

The moderate lawmakers’ threats have only drawn liberals’ ire, since the left-leaning bloc months earlier issued the opposite ultimatum, demanding Democrats secure the budget before tackling infrastructure. With no resolution in clear sight as of Sunday morning, the dispute risked scuttling Pelosi’s plans.

“I will vote against the budget resolution, as we’ve said, as the nine of us committed publicly,” Gottheimer said on Saturday. “We will vote against a budget resolution if the infrastructure package isn’t brought up first.”

In a letter to lawmakers sent late Saturday, Pelosi again made the case for swift action on the budget as well as infrastructure before the end of September. “Any delay to passing the budget resolution threatens the timetable for delivering the historic progress and the transformative vision that Democrats share,” she said.

House Speaker Nancy Pelosi (D-Calif) is facing down threats from the moderate wing of her party, as the House returns for a series of key votes. (Zach Purser Brown/The Washington Post)

Even if the party does ultimately come together, however, some Democrats acknowledged this weekend that the spat has lasting significance — foreshadowing potentially further clashes as lawmakers translate their budget into fuller legislation still to come.

“Is getting there going to be easy? No,” said Rep. Ro Khanna (D-Calif.), the deputy vote counter for the Congressional Progressive Caucus, which has opposed the moderates’ demands. “It’s going to be a challenge. I’m confident we’ll get it done, but there are going to be curveballs and obstacles along the way.”

For now, the budget vote marks the critical next step in Democrats’ vast ambitions to expand the footprint of government to a level not seen in decades. Some of its most defining elements mirror proposals Biden put forward earlier this year as part of his jobs and families plans — and echo commitments that Democratic lawmakers say had helped them capture majorities in Congress during the last presidential race.

The spending blueprint envisions a major injection of new federal spending to help parents and children, including new funds for child care, universal prekindergarten and paid sick leave. On health care, meanwhile, Democrats hope the budget fosters a major expansion of Medicare so that it can cover dental, vision and hearing benefits, along with new prescription drug rules that might lower the cost of medicine for seniors.

Additional spending aims to address the effects of climate change by clamping down on pollution and incentivizing cleaner energy. Democrats say the money is essential, with some arguing the bipartisan infrastructure deal adopted by the Senate falls far short of what is necessary to combat the consequences of a warming planet. And Democrats have tucked into the budget other initiatives to help immigrants obtain lawful resident status, assist workers trying to unionize and ease the financial burden some Americans face in securing housing.

Democrats aspire to finance the proposal through a series of changes to tax laws that raise rates on corporations, investors and wealthy families — unwinding many of the cuts imposed under President Donald Trump. But party lawmakers remain unsettled on the exact size and scope of these tax hikes, reflecting the tough task ahead of them to turn their rough budget outline into legislation. The vote this week would merely unlock the process known as reconciliation, a move that allows Democrats particularly in the Senate to bypass a Republican filibuster.

The process of crafting a bill, then shepherding it through the House and Senate, is expected to span months. And, in the meantime, it is sure to test Biden, Pelosi and other party leaders, whose razor-thin majorities in both chambers of Congress mean there’s little room for error — a political reality that surfaced even before the House took its first vote.

The first signs of discord emerged about two weeks ago, when Gottheimer and his centrist allies first publicly called for infrastructure investments to come before the budget given the immediate economic benefits that the new public-works spending could provide. The moderates concluded their letter to Pelosi with a threat: “We will not consider voting for a budget resolution until the bipartisan Infrastructure Investment and Jobs Act passes the House and is signed into law.”

Without those nine lawmakers, Pelosi and her Democratic allies simply do not have the votes to adopt the budget in the face of sustained, unanimous GOP opposition. But the speaker may not be able to accede to their demands, either, since the nearly 100 members who belong to the party’s Congressional Progressive Caucus previously signaled they would withhold their votes on infrastructure until Congress completes work on its budget. That includes both the outline set for a vote this week as well as the final reconciliation package, as liberals seek to maximize their political leverage and ensure their priorities don’t fall out of a final deal.

Khanna, a top member of the caucus, said Saturday the caucus remains resolute in its stance to “guarantee” that Congress acts on its agenda, including climate change. He described the moderates’ opposition — if they do ultimately fulfill their threat — as a “slap in the face of Joe Biden” since the president has endorsed Pelosi’s strategy.

Gottheimer, meanwhile, faulted progressives and the “far-left” portions of the Democratic Party for jeopardizing an infrastructure package that would improve the nation’s roads, bridges, pipes, ports and Internet connections. Waiting until after reconciliation, he said, could prevent the passage of widely backed public-works spending until late in the fall.

While Gottheimer added that he supports a reconciliation package, he stressed moderates are “holding strong to our principled beliefs.”

Caught between two wings of her own party, the speaker so far has refused to blink. She and other House Democratic leaders urged “unanimity” on a call with the caucus on Tuesday. She issued a public letter that same day, warning that any delay “jeopardizes once-in-a-generation opportunity we face to enact initiatives that meet the needs of working families.” And Pelosi joined top Democratic lawmakers on a call with Biden on Thursday, stressing in a statement later that she and the White House share a “determination to produce results — and soon.”

The White House has sought to aid Pelosi’s efforts, as top aides to the president — including Louisa Terrell and Steve Ricchetti, and a trio of Cabinet secretaries — have called moderates in recent days to hear their concerns yet encourage them to fall in line. A person familiar with the outreach, who spoke on the condition of anonymity to describe the private conversations, confirmed the calls this weekend.

Gottheimer and other moderates on those calls have sought to negotiate a potential end to the standoff, a second source said, without offering specifics. But lawmakers ended last week with no deal in hand, leading the nine centrists to assert their stance Friday in a series of statements that argued for urgent fixes to the country’s inner workings.

“In a deeply divided Congress, it is virtually impossible to pass such major initiatives, and any changes or delays will likely cause this one to fail,” said Rep. Ed Case (D-Hawaii), one of the members of the centrist group. The congressman also led a second effort on behalf of the broader Blue Dog Coalition, calling for an infrastructure vote immediately, though the bloc did not explicitly threaten to withhold its votes.

Pelosi and Biden spoke again Saturday, according to the White House, during which the president “reiterated his support” for her timeline on the budget and infrastructure reform. Pelosi hours later issued a new missive to Democratic lawmakers, stressing it is “essential that our caucus proceeds unified in our determination to deliver once-in-a-century progress for the children.”

Despite the dispute, Democratic leaders in the House and Senate still hope they will prevail — and have tasked key lawmakers with crafting their portions of the bill by Sept. 15. The tight timeline has touched off a frenzy of behind-the-scenes work even before the House officially convenes to adopt the budget this week.

On the House Education and Labor Committee, which oversees a vast swath of the new spending, Democrats have found themselves dusting off old ideas they had tried and failed to adopt when they were in the minority on Capitol Hill. Chairman Robert C. “Bobby” Scott (D-Va.) said his panel is preparing a wide array of proposals to boost child care, offer new family and medical leave and provide prekindergarten to all families, much of which tracks with a slew of bills he and his colleagues have offered in recent years.

“It has the potential of really helping working men and women across the country,” Scott said of the work to come.

But the same divisions that have plagued Democrats as they prepare to take up the budget loom large as the party begins to construct a reconciliation package. Some moderates have raised early alarms with the total $3.5 trillion price tag, once again putting them at odds with liberals, some of whom sought as much as $6 trillion earlier in the debate. Centrist House Democrats share those cost concerns with their counterparts in the Senate, including Sens. Joe Manchin III (D-W.Va.) and Kyrsten Sinema (D-Ariz.), who have signaled they expect to see the final package slimmed down.

Slashing the total size of the package could force Democrats to remove entire programs or lessen the amount of aid they provide toward health care, education and other priorities. Disputes over tax increases to pay for the package, meanwhile, would lessen its revenue — and force Democrats to scale back their spending if they hope to ensure their agenda does not add to the federal deficit.

For now, some top Democrats have stressed in the days before the House vote that they must stay focused on a big, robust reconciliation package, raising the specter of another fight between the party’s various factions over what that should entail.

“This is about making long-term transformational investments. The deck has been stacked for the wealthy and well connected, and working families have been left behind,” said Rep. Rosa L. DeLauro (D-Conn.), the leader of the House Appropriations Committee and a member of the progressive caucus.

“This budget reconciliation bill starts the process in moving forward, and that’s what we will do,” she said, adding of the haggling to come: “There will be a discussion.”