A bipartisan Senate deal to improve the nation’s infrastructure again appeared in political peril Tuesday as Republican negotiators demanded a delay on an upcoming vote on the proposal until next week.

With Senate Majority Leader Charles E. Schumer (D-N.Y.) refusing to bend, GOP senators indicated that they plan to oppose a vote to begin debate on the deal they helped craft. In a closed-door lunch Tuesday afternoon, Senate Republicans came to a consensus that they could not support advancing a package that had yet to be finalized, although negotiators have insisted for days that they are close to putting the finishing touches on the agreement, according an attendee.

“There’s absolutely no reason why [Schumer] has to have the vote” Wednesday, said Sen. Susan Collins (R-Maine). “It does not advance the ball. It does not achieve any goal except to alienate people.”

The new rift emerged less than 24 hours before the chamber was set to take that critical procedural step toward considering a still-forming, roughly $1 trillion agreement to improve the nation’s roads, bridges, pipes, ports and Internet connections. Despite months of work, lawmakers remain torn on how to spend the money — and how, exactly, to pay for that spending — prompting some Republicans to say they need more time.

Sen. Rob Portman (R-Ohio), one of the lawmakers closely involved in figuring out how to finance the package, acknowledged Tuesday that negotiators are “not going to have a product ready” Wednesday in time for the vote.

The vote is mostly procedural. Schumer has said he is seeking only to speed up the chamber’s lumbering legislative process, essentially using a shell of a bill that later could be replaced with a fuller infrastructure deal. But the timeline has nonetheless provoked unease among Republicans, without whom Democrats cannot proceed.

“I think all of the issues will be resolved,” said Sen. Mitt Romney (R-Utah), one of the lawmakers hammering out the deal. He said his preference is that “Senator Schumer will have the vote Monday when we’ve had a chance to resolve any remaining outstanding issues.”

To underscore how close the negotiators are to a final agreement, several GOP senators were readying a formal letter Tuesday to Schumer, saying they would be prepared to vote to advance the infrastructure package Monday, according to people familiar with its contents. At least eight GOP senators as of Tuesday afternoon had already committed to doing so, according to one Republican directly familiar with the effort, and 10 would be needed to advance legislation if all 50 Democratic senators were on board.

The requests came on a day when Democrats across the Capitol expressed growing unease with what they described as the Senate’s seemingly characteristic slowness. Liberal-leaning lawmakers in the House — many of whom have long sought more robust spending than the Senate is likely to adopt anyway — encouraged Schumer to forge ahead despite the Republicans’ latest demands.

“Time’s a-wastin’. We’ve got to get this work done,” said Rep. Pramila Jayapal (D-Wash.), leader of the Congressional Progressive Caucus.

Jayapal said Schumer and other Democrats in the Senate needed to be “very wary” of the Republicans’ tactics. “We have all the history in the world to show that this is what Republicans do time and time and time again,” she said.

The angst further illustrates the delicate politics surrounding the infrastructure debate — and the tough slog ahead for Democrats who seek to deliver on President Biden’s roughly $4 trillion economic agenda. For all the bipartisan agreement over the need to invest heavily in the country’s inner workings, there are still serious schisms that threaten to keep lawmakers from securing the bipartisan victory that both sides insist they want.

Schumer initially scheduled the Wednesday procedural vote as part of a broader commitment to move infrastructure legislation and a roughly $3.5 trillion budget deal through the Senate before lawmakers depart for their summer recess. In a nod to the work still underway to craft the bipartisan public-works deal, the Democratic leader said he would see to it that the Senate started debate only on legislation it essentially had already adopted — including recently passed bills that put billions of dollars toward the country’s roads, bridges and pipes.

“It is not a final deadline for legislative text,” Schumer said Tuesday. “It is not a cynical ploy. It is not a fish-or-cut-bait moment. It is not an attempt to jam anyone.”

Schumer’s timeline still has functioned as an inflection point, injecting new urgency into negotiators’ work to fill in the finer details of a roughly $1 trillion agreement they announced in principle a month ago. Some Republicans said they expect to see full legislative text — and an official analysis of its budget implications — before they can vote to proceed.

Behind the scenes, negotiators have scrambled to find alternative ways to pay the costs of their package. Their latest idea includes changes to Medicare rules proposed under President Donald Trump that generally prevent drug companies from providing rebates to pharmacy benefit managers and insurers that cover their medicines. The rule would have incurred a cost, so its repeal could help raise as much as $200 billion in revenue for infrastructure, according to past congressional budget reviews.

The revenue theoretically could help cover the gap created after lawmakers had to withdraw one of their initial plans to pay for public-works spending by empowering the IRS to pursue unpaid federal taxes. Republicans had balked at that idea, arguing that even some of the more limited changes proposed to augment the staff of the tax-collection agency could lead to abuse.

Sen. Jon Tester (D-Mont.), one of the lawmakers helping to craft the deal, acknowledged Tuesday that a change to the rebate rule is part of lawmakers’ latest thinking. Negotiators were set to meet again Tuesday afternoon, Tester said, expressing fresh hope that potentially “we can wrap it up.”

Romney, meanwhile, said discussions remained underway on potentially repurposing some coronavirus aid dollars to cover the cost of infrastructure investments — an idea that in the past has given the White House pause.

“I’d like to make sure that we have a chance to explain what’s in the bill to our colleagues and resolve any outstanding issues, and I’ll see if we can’t get some people to support it,” Romney said in making the case for delay.

With the fate of the proposal anything but guaranteed, the uncertainty added to the frustrations among House Democrats, who expressed fears that the talks are dragging on seemingly without an end in sight.

“I mean, they’ve been killing time for months,” said Rep. Alexandria Ocasio-Cortez (D-N.Y.). “And at this point, I believe that it’s starting to get to a point where this bipartisan effort is seeming to serve less on investing in our infrastructure and serving more the end of just delaying action on infrastructure. It has been enough. It has been months.”