Senate Majority Leader Charles E. Schumer (D-N.Y.) on Thursday sought to move lawmakers one step closer to debating and potentially passing roughly $4 trillion in new federal spending, announcing his plans to hold a key chamber vote on a still-forming infrastructure deal next week.

The announcement essentially doubled as a major new deadline for a group of roughly two dozen Democrats and Republicans, who for the past few months have been hammering out a proposal to invest close to $1 trillion in the country’s roads, bridges, pipes, ports and Internet connections.

Schumer delivered the new timetable on a day when negotiating lawmakers struggled to reconcile their many policy differences, particularly over how to pay for the level of spending they support. The schisms bore out over the course of a roughly three-hour meeting with top White House aides, highlighting the challenges that Senate leaders face in finishing their work ahead of Schumer’s scheduled Wednesday vote to start debate on infrastructure legislation.

“The good news is we are all still talking. The good news-bad news is we’ve got a pretty tight time frame,” Sen. Lisa Murkowski (R-Alaska) said as she exited the private gathering. “There are details we have to resolve, and those details involve things like [paying for it].”

One of the key components financing their tentative legislation — increased federal efforts to collect unpaid taxes — seemed in particular political peril Thursday amid sustained GOP opposition. Its potential absence from the final package left lawmakers scrambling to find other ways to cover the costs of their long-sought infrastructure investments, because some senators from both sides have maintained that they are not willing to support an agreement that adds to the deficit.

“You’ve got to pay for it. We’re not going to pass it unless we do,” acknowledged Sen. Bill Cassidy (R-La.), another participant in the talks.

In calling for the early infrastructure vote next week, Schumer on Thursday also issued a similar edict to his own caucus, telling Democrats that they need to come to a full agreement on a second package that includes additional spending to enact President Biden’s fuller economic agenda.

Democrats announced Tuesday the outlines of that deal — a $3.5 trillion budget measure that proffers major changes to health care, housing, climate policy and the tax code — after weeks of haggling. The proposal is expected to fund programs left out of the emerging infrastructure measure because they do not garner support from Republicans.

Democrats intend to sidestep that GOP opposition on their budget deal by forging ahead with the package using a process known as reconciliation, which requires only a simple majority, not the usual 60 votes in the Senate, to pass. Schumer has described this as a key element of a two-track process to deliver on the White House’s economic priorities, stressing Thursday that the caucus needed to “agree to move forward” with the budget, the first step in what is sure to be a lengthy policy battle.

The two deadlines injected fresh uncertainty into what already has been a fragile congressional debate. Even though lawmakers in both parties agree on the need to invest anew in the nation’s infrastructure, they remain divided over how, exactly, to do it — including how much to spend and the ways in which they should finance it.

But Democratic leaders still maintain that they hope to secure passage of an infrastructure bill and a budget agreement before departing for their usual summer recess. They set their ambitious timetable in motion a day after Biden visited Democratic lawmakers in the Capitol and implored them to work together to adopt spending that many party leaders see as transformational.

Speaking on the Senate floor, Schumer echoed the president’s vision, describing the two endeavors as the “most significant legislation to expand support for Americans families since the era of the New Deal and the Great Society.”

“It’s important to keep the two-track process moving,” he said.

First, though, lawmakers need to actually craft an infrastructure bill.

In the hours after Schumer spoke, Senate Democrats and Republicans resumed huddling in private over their plan, the latest in a line of frenetic meetings this week to transform an outline for $1 trillion in spending from June into actual policy. The Democratic leader’s announcement immediately added new uncertainty to the talks, prompting some lawmakers, including Sen. Mike Rounds (R-S.D.), to fret the new “artificial deadlines.”

While the Senate essentially can start debate before it finishes writing a bill, some Republicans expressed deep misgivings about advancing the process prematurely. In the meantime, lawmakers from both parties acknowledged they had considerable differences still to resolve — including immense political hurdles over the financing in their nascent deal.

One of the key mechanisms to pay for it had been heightened IRS enforcement of federal tax laws. The idea generally has garnered support in the Capitol, as lawmakers look to recoup as much as $1 trillion in unpaid taxes. But the agency and its track record historically has left even some participating Republicans uneasy — though lawmakers crafting the deal said they hoped to put strong safeguards in place to prevent abuse.

Those safeguards still seemed insufficient by Thursday, prompting Senate aides, who spoke on the condition of anonymity to describe the private talks, to say it will probably be removed from the final legislative package. Lawmakers including Sen. Jon Tester (D-Mont.), who joined the talks Thursday, acknowledged the IRS reforms remain unsettled — and noted that work continues to identify potential alternatives that senators as well as the White House would support equally.

At a news conference at the White House later Thursday, Biden acknowledged some “last-minute discussion" over how to pay for both of the bills being crafted in the Senate. But he stressed that he believes “we will get it done."

“I’ve watched and listened in the press declare my initiative dead at least 10 times so far,” Biden said in a joint news conference with German Chancellor Angela Merkel, who was visiting the White House on Thursday. “I don’t think it’s dead. I think it’s still alive.”

Still, the impasse meant lawmakers departed the Capitol in the early afternoon with no final agreement in hand — and no guarantee they would resolve their disputes in the weekend, either. Yet Democrats and Republicans alike still said they were committed to reach consensus.

“I don’t know if we’ll make anybody’s arbitrary timeline,” said Sen. Rob Portman (R-Ohio), who left the gathering well after White House aides and other lawmakers departed. “But that’s not the point — the point is to get it right.”