Senate Democrats are aiming to vote and approve a roughly $2 trillion package to overhaul the nation’s health-care, education, climate, immigration and tax laws before Christmas, hoping to muscle through a jam-packed schedule to deliver the remaining piece of President Biden’s economic agenda.

Writing to lawmakers on Monday, Senate Majority Leader Charles E. Schumer (D-N.Y.) affirmed the aggressive timeline, warning that there are “more long days and nights” on the horizon as the chamber races to resolve a wide array of fiscal and economic issues before the end of the year.

“This is arduous work. It takes time, precision and a lot of pieces moving together,” he later said on the Senate floor.

Yet the path to passage for Democrats’ signature spending plan appears especially precarious, as party lawmakers continue to contend with political dissent among their own ranks. A pivotal swing vote, Sen. Joe Manchin III (D-W.Va.), has yet to offer his endorsement of the legislation, even after months of wrangling with the White House.

On Monday, Manchin signaled again that he harbors “concerns” with the size and scope of the bill. And he affirmed his trepidations about advancing so much new spending at a time when inflation continues to raise the price of goods, which Manchin said reflected an economy that is “vulnerable” to other disruptions.

“We’re talking about major changes in our tax code, we’re talking about [a] major overhaul of our social [programs], and we’re talking about a tremendous overhaul of our climate positions that we have,” the senator said.

The $2 trillion proposal, known as the Build Back Better Act, aims to expand Medicare coverage, invest new sums to combat climate change, authorize universal prekindergarten and provide new aid to low-income families, all financed through tax hikes targeting rich Americans and corporations. House Democrats adopted the bill in November, teeing it up for the Senate, where party lawmakers at times have been divided over its size and scope.

President Biden is hitting the road to tout the passage of the infrastructure bill, hoping he can reverse his flagging poll numbers before the 2022 midterms. (Joyce Koh/The Washington Post)

From here, the Senate still must rejigger critical parts of the bill to ensure it is compatible with the process known as reconciliation. The legislative maneuver allows Democrats to approve the legislation with 51 votes, rather than the usual 60, sidestepping a guaranteed Republican filibuster in the narrowly divided chamber.

But reconciliation carries its own set of potential headaches, as Democrats must ensure every element of their sprawling tax-and-spending proposal directly implicates the federal budget — or else it is at risk of being stripped out of the measure entirely. Anticipating those issues, lawmakers have been meeting behind the scenes with the chamber’s parliamentarian, a customary process that Schumer said is expected to continue “this week and next.”

In the meantime, Democrats have not settled on some of the finer details of the bill itself. Manchin continues to battle with lawmakers over the inclusion of a program to provide paid family and medical leave to millions of Americans. And other party lawmakers are locked in a still-unresolved dispute over a state-and-local tax proposal that some liberals see as too generous to the wealthy.

The wrangling over the bill only reflects the vast work ahead of Congress just 25 days before the end of the year. Democrats and Republicans still have to approve a bill that would authorize nearly $778 billion in defense spending, for example, which has been mired in bitter disputes around U.S. policy toward China and Russia.

Some lawmakers also have discussed using the annual Pentagon measure to address the unrelated yet critical issue of the debt ceiling, which permits the country to borrow to pay its bills. Congress has nine days until the U.S. government may begin to face difficulty issuing new debt, according to the Treasury Department, though other analyses have said Washington has more time until it crosses that dire fiscal threshold.

Speaking to reporters Monday, House Majority Leader Steny H. Hoyer (D-Md.) said he hopes the chamber can address the debt limit as soon as this week. But he cautioned that the issue is “up in the air,” as Democrats and Republicans work out a potential deal in the Senate. Lawmakers from both parties in recent weeks have labored to avert another political showdown, after GOP lawmakers initially refused to supply votes in the narrowly divided chamber as part of a broader opposition to Biden’s spending priorities.

Schumer, for his part, said Monday that Democrats plan to address the issue soon — declining to offer additional details. But he praised Senate Minority Leader Mitch McConnell (R-Ky.) for engaging in productive talks, raising the prospect that Congress could come to a resolution on the debt ceiling well before the debate can wreak any political and economic havoc.

In setting a Christmas deadline for the Build Back Better Act, meanwhile, Schumer sought to continue a strategy of keeping pressure on the Senate, a chamber that isn’t exactly known for swift action. The timing is significant, since the proposal extends a key aid program — expanded, monthly child tax credit — that is set to expire at the end of the year. Unless Congress takes action, millions of Americans will receive their final payments this month, a possibility that prompted some lawmakers on Monday to call on the Senate to make haste.

“American families cannot afford to lose this critical middle-class tax cut, which has cut child poverty in half and helped millions of families afford child care, pay their bills, and put food on the table,” said Rep. Suzan DelBene (D-Wash.), the chair of the centrist-leaning New Democrat Coalition, which has pushed for the provision.

Biden himself sounded an eager yet patient note Monday. Asked whether lawmakers can accomplish Schumer’s goal, adopting the spending bill before holiday, he briefly told reporters: “As early as we can get it. We want to get it done no matter how long it takes.”

Marianna Sotomayor contributed to this report.