House Speaker Nancy Pelosi on Monday sought to quell an internal Democratic revolt over President Biden's economic agenda, as growing rifts within the party threatened to scuttle a planned vote this week on a roughly $1 trillion package to improve the nation’s infrastructure.

The speaker delivered her pleas at a private, roughly 90-minute caucus meeting, where she and a roster of congressional allies — including some of the House’s most vulnerable candidates — made the case for unity among the party’s increasingly discordant liberal and moderate ranks.

Centrist Democrats have demanded a vote on the proposal to improve the country’s roads, bridges, pipes, ports and Internet connections, which Pelosi (D-Calif.) has tentatively set for Thursday. More-liberal Democrats, meanwhile, have threatened to oppose the public-works measure, hoping to use their support as leverage to secure a second, roughly $3.5 trillion package that moderates for months have tried to whittle down. That legislative effort aims to raise taxes on wealthy Americans and corporations to pay for a number of new programs to expand health care, improve education and combat climate change.

Addressing the caucus on Monday, Pelosi and other Democratic leaders doubled down on their timetable, aiming to hold the infrastructure vote as planned. But she backed off an earlier attempt to vote this week on the $3.5 trillion package as well, citing the fact that lawmakers continue to negotiate its contours, particularly with centrists led by Sens. Joe Manchin III (D-W.Va.) and Kyrsten Sinema (D-Ariz.).

Pelosi did warn that the package is likely to be smaller than the $3.5 trillion price tag that some Democrats initially anticipated, according to multiple lawmakers who attended the meeting who spoke on the condition of anonymity to describe the private affair. In doing so, the speaker and other leaders also urged for patience, stressing the political significance of their work — with potentially Democrats’ majorities in the 2022 election on the line.

Many of the Democrats' messages echoed Biden, who touted his agenda with a note of cautious optimism during an event earlier Monday.

“I think things are going to go well. I think we’re going to get it done,” the president said, later adding: “It may not be by the end of the week. I hope it’s by the end of the week.”

The roiling divisions only add to the challenges that House and Senate Democrats face in the days ahead. Party leaders also must address a series of looming fiscal deadlines — including an urgent need to fund the government before Friday, at which point some federal agencies and programs are set to shut down. Senate Republicans on Monday dealt those efforts another blow, as they voted to block their chamber from adopting a short-term funding measure that also would have provided new hurricane aid and raised the debt ceiling.

As a possible shutdown looms later this week, House Democrats have tried to forge ahead in advancing roughly $4 trillion in new spending initiatives long sought by Biden. The first step in that long legislative slog came Monday, as the House began debating the roughly $1.2 trillion infrastructure plan that cleared the Senate last month, with the goal to bring it to a final vote Thursday.

With the outcome in doubt, the two warring Democratic camps began Monday reiterating their past threats. A group of centrists led by Rep. Josh Gottheimer (D-N.J.) urged Pelosi to keep her promise to move much-needed infrastructure investments swiftly.

“The American people have waited long enough for the jobs and investment this bill will deliver,” they said in a joint statement.

And Rep. Pramila Jayapal (D-Wash.), the leader of the Congressional Progressive Caucus, estimated there are 60 Democrats who would oppose the infrastructure bill if it sees a vote before Democrats finalize their $3.5 trillion plan. Some of the members of her bloc even went as far as to demand the Senate vote on the measure before they advance infrastructure.

With tensions running high, the threat of potential defections is significant, especially in the narrowly divided House: Pelosi can afford to lose only three Democrats in the chamber, and Republicans are unlikely to help pass the infrastructure bill even if they support it in principle, as they try to deliver Biden a defeat.

The feuds have ensnarled the White House in recent days, prompting Biden personally to intervene to try to save the bulk of his first-term agenda from collapse. He gathered moderate and liberal lawmakers at the White House last week, with talks that continued into the weekend, according to a White House official who spoke on the condition of anonymity to describe the president’s involvement. The aide said that there is “strong resolve” throughout the caucus for passing the two bills.

The tenuous political dynamic also set the stage for Pelosi to canvass her caucus Monday, stressing that the House hopes to reach a deal with moderates in the Senate around the future of their tax-and-spending package — hopefully with enough detail to lessen liberals’ blockade.

One person in the room said Pelosi held firm on the reality that the $3.5 trillion bill is going to have to be pared down, reminding people that a number of priorities may not be compatible with the strict rules that govern the process in the Senate. Democrats aim to adopt the bill using a maneuver known as reconciliation, which allows them to sidestep GOP opposition — but limits their bill to measures that directly affect the budget.

The goal, Pelosi said, is for the House to craft its reconciliation bill in a way that survives Senate scrutiny, from skeptical lawmakers and the parliamentarian alike, a source said. That, the speaker added, would ensure that the measure isn’t whittled down in a way that could upset liberals.

Some of the House Democrats’ most vulnerable 2022 candidates, colloquially known on the Hill as “frontliners,” urged the need for unity and stressed they share their goal of passing Biden’s agenda. They said that failure to pass either bill would be much more consequential of a loss, according to those who attended the meeting.

But the pleas still left some Democrats across the ideological spectrum uneasy, even as the clock ticked ahead of a scheduled Thursday vote.

Speaking to reporters after Democrats met late Monday, Jayapal stressed that her bloc of roughly 100 lawmakers is still open minded about the assurances they receive. In a sign that distrust remains high, though, the congresswoman said that the “whole bill has to be agreed upon.”

Jacqueline Alemany contributed to this report.