WASHINGTON — A key Republican senator grumbled publicly that Democrats were not being responsive enough. One aide anonymously described a litany of unresolved disagreements, while another privately put out a list of broken promises. And former President Donald J. Trump weighed in from afar, urging Republicans to walk away from the talks, lest they be played by Democrats as “weak fools and losers.”
Nearly five weeks after President Biden and a group of Republican and Democratic senators triumphantly announced that they had risen above the polarization gripping Washington to agree on a framework for an infrastructure deal, there was trouble in bipartisan paradise on Monday as their agreement appeared to teeter on the brink of collapse.
With just two weeks left before a scheduled monthlong August break, the efforts of the bipartisan group to translate their plan into a detailed bill had reached that most excruciating of stages: tantalizingly close to an agreement but with none in hand. And patience was wearing thin.
Senator Susan Collins, Republican of Maine and a key negotiator, said she believed the deal would “ultimately” get done. “But it’s painful,” she added.
Having already dangled the possibility of delaying the recess as a motivational threat, Senator Chuck Schumer, Democrat of New York and the majority leader, warned colleagues about the prospect of working through the weekend. He has said he intends to hold votes before the August break on both the nearly $600 billion infrastructure deal and a $3.5 trillion budget blueprint set to carry much of the rest of Mr. Biden’s economic agenda.
The mood on Capitol Hill was grim, as private policy disagreements turned into public finger-pointing between the two parties. And a group that had been held up just days ago as the best chance for bipartisan success in a divided Congress was itself veering into dysfunction, with many senators tired of negotiating.
Perhaps it was an indication that the talks were just about to yield fruit, wallowing in the darkest moments of disagreement before a dawn when negotiators would work it all out. But it seemed just as likely they were headed toward collapse, confirming the expectations of legions of skeptics.
“I think they’re down to where it all seems easy until you get to the final details,” said Senator Roy Blunt of Missouri, a member of Republican leadership. “Then they’re never as easy as you think they’re going to be.”
In the meantime, aides traded accusations — on condition of anonymity, naturally, to avoid jeopardizing the chances of a deal — about who was at fault for the impasse. The laundry list of unresolved items ballooned. There was talk of unserious offers rejected, self-imposed deadlines discarded and goal posts being moved.
Leaving a meeting Monday evening, senators involved in the talks said conversations would continue.
“I thought that they’d reached an agreement — I thought they were that close, they were down to one or two points,” Senator Richard J. Durbin of Illinois, the No. 2 Democrat, lamented. Then, referring to the Senate Republican leader, he added: “But keep in mind that the longer it goes, Mitch McConnell’s hand gets stronger, and is able to pick out this part and that part, and try to peel off one of the Republican senators here and there. And then here comes Donald Trump with his declaration today.”
If things were not precarious enough, Mr. Trump, who as president blew up bipartisan infrastructure discussions and pulled the plug on stimulus talks, issued an unsolicited condemnation of the emerging deal, putting pressure on Republicans to abandon the negotiations and handing Democrats fodder for the emerging blame game.
Senator Rob Portman, Republican of Ohio and a lead negotiator, said on Sunday that Democrats were not responding to his side’s proposals. Other Republicans accused Democrats of derailing the talks by attempting to revisit provisions that had already been finalized and agreed to. One Republican aide circulated a list of agreements on which Democrats had supposedly reneged, including how to finance the plan and whether to implement wage requirements for new infrastructure projects.
But the negotiators, having weathered months of skepticism over their pursuit, remained adamant that there was a deal to be had. Given the complexity of the topics at hand, they chided reporters for impatience and pessimism and insisted the outlook was better than it seemed.
“How many times have I said this? I still think there’s a chance we get it done today,” Senator Jon Tester, Democrat of Montana and perpetually the sunniest of the group, told reporters. “I feel very bullish it’ll be done by morning.”
“Give me some positive vibes,” he said later, as he left a negotiating session on Capitol Hill.
On Sunday night, Democrats had delivered an offer intended to resolve a series of sticking points, including funding for water infrastructure, highways, bridges and broadband, as well as using unspent coronavirus relief funds to finance the package.
Part of that offer included a proposed trade-off, according to a Democratic official close to the negotiations, that Democrats would accept the amount of highway funding proposed by Republicans if the G.O.P. would agree to provide more transit funding. But Republicans declared the overall offer insufficient on Monday.
The contents of the agreement have remained shrouded in mystery to all but the negotiators as details shift. The framework the White House released last month is now out of date, according to people familiar with it, as a key financing provision has been gutted and replaced.
Negotiators met again Monday evening, as they returned to Washington from another weekend spent haggling from afar. The core bipartisan group of 10 senators and White House officials has been working to translate their agreement in principle — expected to provide $1.2 trillion over eight years, with nearly $600 billion of that in new federal funds — into legislative text.
After Republicans unanimously blocked the Senate last week from taking up the emerging plan, it was unlikely that Democrats would attempt to bring it up again without clear buy-in from enough Republicans to clear the 60-vote threshold and overcome a filibuster.
Even if the group reaches agreement and the deal passes the Senate, its path forward is complicated. Speaker Nancy Pelosi of California has said she will not bring it up for a vote in the House until the far more ambitious budget blueprint, which Democrats plan to muscle through unilaterally over Republican opposition, has been approved.
For now, though, the group is still trying, however arduous the process. On Monday evening, Senator Mark Warner, Democrat of Virginia and a lead negotiator, offered his answer to an ESPN query on Twitter about things that feel like an Olympic sport, but are not.
“Trying to negotiate a bipartisan infrastructure deal,” Mr. Warner replied.
Nicholas Fandos, Jim Tankersley and Catie Edmondson contributed reporting.