WASHINGTON - The clock is ticking on House Democrats as they barrel toward an uncertain, scheduled vote Thursday on a $1 trillion bill to improve the nation's infrastructure - a top priority of President Joe Biden that some in his party are still threatening to oppose.

The drama and diplomacy only have intensified over the past 24 hours, as House Speaker Nancy Pelosi, D-Calif., scrambles to keep her fractious, narrow majority intact and send the first of two major economic initiatives to Biden's desk. But her last-minute push - along with the president's own flurry of outreach - appears to have made little difference in sparing their shared agenda from the risk of a high-profile collapse.

Democrats generally support the infrastructure package, which proposes major new investments in the country's aging roads, bridges, pipes, ports and internet connections. But the bill has become a critical political bargaining chip for liberal-leaning lawmakers, who have threatened to scuttle it in order to preserve the breadth of a second, roughly $3.5 trillion economic package.

At issue for the left-leaning lawmakers are the demands issued privately and publicly by moderates, including Sens. Kyrsten Sinema, D-Ariz., and Joe Manchin III, D-W.Va., who have sought to whittle down the price tag and policy scope of the $3.5 trillion measure. The proposal as it stands aims to expand Medicare, combat climate change, offer new benefits to low-income students and families and raise taxes on wealthy Americans and corporations - a series of long-sought Democratic priorities that liberals have vowed to preserve.

By Wednesday afternoon, however, no resolution to the standoff appeared to be in sight. Sinema continued to huddle with Biden's top aides, hoping to strike an agreement to reduce the $3.5 trillion package, yet a final number still had yet to materialize. Manchin, meanwhile, cast doubt on the possibility that any such deal even could come together in time for the House's scheduled Thursday vote on infrastructure.

"No, it's not possible," he told reporters.

Absent a deal, the political dynamic threatens to leave Pelosi facing a difficult choice in the hours ahead - putting the infrastructure bill on the floor at the risk of a rare defeat, or pulling it back in a move that could upset moderates among her own ranks. For now, though, Democratic leaders doubled down, stressing they intend to stick with their original plan.

"I hope there's an infrastructure - I expect an infrastructure vote tomorrow," said House Majority Leader Steny Hoyer, D-Md., on Wednesday.

The tense and tentative mood on the Capitol only served to illustrate the high stakes for Democrats, who campaigned in 2020 on the promise of big, bold investments - and now face pressure to deliver them with the midterm elections about a year away. Pelosi herself has touted the consequences of failure in private meetings with her own caucus this week, reflecting the extent to which the party's narrow but powerful House and Senate majorities are on the line.

Adding to their headaches, Democrats also must address a series of looming fiscal deadlines. The House and Senate plan to vote Thursday on a bill that would prevent a government shutdown Friday. And they are still grasping for a way to raise the debt ceiling by mid-October as Democrats and Republicans are at odds over how to lift the borrowing cap.

"With so many critical issues to address, the last thing the American people need right now is a government shutdown," Senate Majority Leader Chuck Schumer, D-N.Y., said Wednesday.

House lawmakers began considering infrastructure legislation on Monday as a result of an agreement brokered between Pelosi and a small group of centrist Democrats led by Rep. Josh Gottheimer, D-N.J. To satisfy liberals, the speaker also initially tried to advance the $3.5 trillion tax-and-spending bill this week, keeping the two proposals linked as she first promised this spring. But Pelosi soon backed down from those plans amid uncertainty in the Senate, where Manchin and Sinema have made clear they still do not intend to support a bill with that price tag.

Speaking to reporters late Tuesday, Gottheimer praised Pelosi and her prowess at whipping votes, expressing a measure of confidence that the infrastructure package can pass even in the face of sustained threats from liberal lawmakers. A day later, he boldly predicted at an event hosted by Politico: "We'll be drinking a nice glass of champagne over here."

But the fate of that infrastructure vote rests on work underway at the opposite end of Pennsylvania Avenue, where Biden increasingly has played the role of political emissary - engaging Sinema and Manchin directly in a series of meetings that continued into Wednesday. The president has pursued a final agreement on a package of spending the two Democrats can support in addition to the infrastructure bill, hoping a deal is enough to satisfy enough liberals and ensure a positive outcome Thursday.

"We're waiting for a couple of people in the Senate to tell us what they're for. That's what the president asked for last week," said Rep. Mark Pocan, D-Wis. "Once they do, we can call this up I think pretty quickly."

For the second time in two days, however, Sinema and Manchin on Wednesday offered little to no detail about their thinking, including how much they would be willing to spend. At the White House, press secretary Jen Psaki described the talks at one point during her daily press briefing as "precarious" - prompting reporters to press her on exactly what she meant.

"You've all asked me some very good and excellent questions about what's going to happen tomorrow," Psaki said, noting at one point Wednesday she does believe Sinema seeks a deal. "And I can't give you a concrete prediction of that."

Their sustained silence only added to liberals' growing frustrations on Capitol Hill. Rep. Ilhan Omar, D-Minn., on Wednesday fretted there's still "no clarity in what (moderates) actually want," joining a chorus of members in the Congressional Progressive Caucus that have pilloried Manchin and Sinema in recent days. Rep. Cori Bush, D-Mo., meanwhile, said she likely would have "no choice" but to oppose infrastructure as a result.

"There is just no way I can tell the people who need these services, who need these resources that you know what, we are going to continue to throw you crumbs, because this is what happens," Bush said.

Their opposition ultimately threatens to sink the public-works bill in a chamber where Pelosi can only afford to lose three votes, raising the specter of a high stakes embarrassment for both the speaker and Biden on Thursday.

Pelosi did raise the possibility that she still could change course, citing at one point Wednesday the powers she has potentially to delay a vote. "Anything that strengthens the hand of the speaker helps pass the bill," she told reporters earlier in the day.

In a sign of the political vise she faces, however, Rep. Stephanie Murphy, D-Fla., the leader of the centrist Blue Dog Coalition, said it would be a "breach of trust" if Pelosi reversed course.

Should that happen, Murphy added, it could "slow the momentum in moving forward and delivering the Biden agenda."

The Washington Post's Annie Linskey contributed to this story.