President Biden’s economic agenda is set to face a major test on Capitol Hill this week, as the Senate barrels toward an early vote on a roughly $1 trillion proposal to improve the nation’s infrastructure even though negotiators still haven’t agreed on key details.

Despite months of frenetic talks, lawmakers are slated to return to the Capitol on Monday in the same political position in which they departed last week: They broadly support new spending to upgrade the nation’s roads, bridges, pipes, ports and Internet connections, yet remain plagued by schisms over how to finance the still-forming package.

Seeking to keep their coalition of about two dozen Democrats and Republicans intact, congressional negotiators in recent days said they opted to eliminate one of the key elements that financed their initial infrastructure blueprint. Senators had hoped to raise about $100 billion by empowering the Internal Revenue Service to pursue unpaid federal taxes, but Republicans balked at the idea out of a concern it would give the tax-collection agency too much power to scrutinize families’ and corporations’ finances.

Without that provision, Democrats and Republicans were left scrambling into the weekend to try to identify alternative ways to pay for their infrastructure blueprint. The meetings continued Sunday, according to Sen. Rob Portman (R-Ohio), one of the lawmakers leading negotiations over the infrastructure deal, who acknowledged the “pushback” they faced on IRS provisions — and stressed the importance in the eyes of many lawmakers that the final product is ultimately “paid for.”

In the meantime, though, the clock continued to tick on their talks: On Monday, Senate Majority Leader Charles E. Schumer (D-N.Y.) is planning to set in motion a Wednesday procedural vote to begin debate on the infrastructure proposal. With the package still in flux, the deadline greatly troubled Republicans, some of whom blasted Democrats for rushing already fragile negotiations.

“Unless Senator Schumer doesn’t want this to happen, you need a little bit more time to get it right,” Sen. Bill Cassidy (R-La.) stressed during an interview on Fox News Sunday.

Portman, appearing on CNN’s “State of the Union,” sounded a similar note of concern.

“Chuck Schumer, with all due respect, is not writing the bill. Nor is Mitch McConnell, by the way. So that’s why we shouldn’t have an arbitrary deadline of Wednesday,” he said. “We should bring the legislation forward when it’s ready.”

The last-minute flurry of negotiations threatened to bring about yet another dramatic week on Capitol Hill, where Schumer has sought to steer the Senate toward adopting an infrastructure proposal in the few short weeks before lawmakers depart for their summer recess. Schumer had promised to put the chamber on that timeline for weeks, yet his decision nonetheless has left little room for error in a Capitol that isn’t known for acting expeditiously.

In recent days, many Republicans have said they may not be willing to support a vote to begin debate on infrastructure reform in the absence of a final agreement or clear legislative text. In doing so, their tentative demands have illustrated the tough task both parties face in building a 60-vote bipartisan coalition in the narrowly divided chamber.

“I think we’ll move quickly, but we’re not going to vote on something until we actually have a bill,” Sen. Mitt Romney (R-Utah), one of its lead negotiators, said before departing the Capitol last week.

Adding to the headaches, the roughly $1 trillion public works blueprint is only one piece of a bigger fight: It is one component of a much larger tranche of about $4 trillion in spending that encompasses Biden’s fuller economic agenda. Even as they negotiate with Republicans over infrastructure, Democrats have opted to seek a second package on their own that aims to boost federal safety-net programs, combat climate change and expand Medicare coverage.

Schumer has described the two efforts as part of an intertwined “two track” process that must move in tandem because many Democrats are not willing to adopt an infrastructure bill without additional spending on their favored priorities. Party lawmakers plan to sidestep expected GOP opposition to the budget deal through a process known as reconciliation, which will allow them to approve it with a similar majority, rather than the 60 votes typically required in the Senate. To that end, Schumer has given his caucus its own loose Wednesday deadline to come together on that proposal as well.

Senate Majority Leader Charles E. Schumer (D-N.Y.) on July 13 signaled an infrastructure reconciliation package could be fully paid for to increase support. (The Washington Post)

“Everyone has been having productive conversations and it’s important to keep the two-track process moving,” Schumer said Thursday in the hours before the chamber departed for the week. “All parties involved in the bipartisan infrastructure bill talks must now finalize their agreement so the Senate can begin considering that legislation next week.”

For now, though, the agreement on infrastructure isn’t yet finalized.

Exiting the Capitol last week, Democrats and Republicans alike acknowledged lingering differences especially in how they intend to finance the roughly $1 trillion in spending they support. Some members from both parties have expressed unease about adding to the deficit, even if they say infrastructure reforms could help spur economic growth, adding to the pressure on negotiators to come up with credible ways to cover the cost of their long-sought upgrades.

The IRS provisions in particular have been a sticking point, as Republicans reject the sort of vigorous enforcement that might be necessary for the federal government to recover as much as $1 trillion in unpaid federal taxes. Privately, Portman and others had sought to reassure GOP lawmakers by proposing guardrails on the IRS’s policing powers. But the ideas did not appear to be enough, forcing negotiators to drop it from their package — though some Republicans later blamed Democrats for the standoff.

Portman on Sunday pointed to Democrats’ decision to endorse what he described as “a lot more” IRS enforcement as a way to pay for their $3.5 trillion budget reconciliation package. He hinted the move had created “quite a problem” for Republicans who already were on the fence about the idea. Going forward, Portman added they are exploring other ways of financing infrastructure reform, including an idea to roll back a potentially costly change to Medicare prescription drug rules announced under the Trump administration.

Much as with IRS enforcement, though, Democrats last week signaled they are eyeing a similar financing mechanism as part of their reconciliation package, which no Republicans have said they are likely to support. The haggling left some GOP lawmakers miffed, blaming Democrats and the White House for the current inability to reach compromise.

“We’ve offered pay-fors, Republicans have, good faith pay-fors that both sides agree are good,” Cassidy said. “And we offered them, and the White House takes them and says, ‘No, we want them. We want them for our $3.5 trillion.’ ”

The White House did not respond to a request for comment.