Congressional lawmakers are set to return to the Capitol on Monday facing a pivotal week for the future of infrastructure reform, as bipartisan negotiations continue and work proceeds to advance a flurry of bills to improve the nation’s roads, bridges, pipes, ports and Internet connections.

At the center of the debate is an infrastructure compromise brokered by 10 Senate Democrats and Republicans. The bloc, largely composed of moderates, faces the new, tough task of selling their deal to both fellow lawmakers and the White House, just days after talks between President Biden and another group of GOP leaders reached a political impasse.

“We're talking to folks, one by one, and just asking folks to be open,” said Sen. Mitt Romney (R-Utah) in advance of the new plan's release.

Some Democrats already have expressed discomfort with the early details of the nearly $1 trillion, five-year package, arguing it should be bigger and more robust in scope. Republicans, meanwhile, signaled there may not be widespread support for it within their own party, either. The White House said at the end of last week it has “questions” about lawmakers’ approach, as fresh concerns emerged over the potential changes to the gas tax that could help finance the new proposal.

But congressional Democrats have said they are not willing to wait much longer in courting Republicans. They’ve already started laying the groundwork to proceed on infrastructure potentially on their own, relying on a process known as reconciliation that might allow them to advance their favored fixes with only 51 votes, not the 60 that are typically required in the nearly deadlocked Senate.

Senate Majority Leader Charles E. Schumer (D-N.Y.) on June 8 said Democrats may pass some infrastructure measures via budget reconciliation. (The Washington Post)

Even though some in the party are not yet comfortable with the move, Majority Leader Charles E. Schumer (D-N.Y.) stressed the goal is to forge ahead on “two tracks” so that lawmakers can keep to their stated timeline of tackling infrastructure in July.

“I’m very impatient,” said Schumer’s top lieutenant, Sen. Richard J. Durbin (D-Ill.), before the chamber adjourned last week.

With the calendar quickly evaporating — and, by Durbin’s count, only six scheduled workweeks between now and September — the party’s chief vote-counter said Democrats could not afford to stomach much more delay.

“I’ve seen this movie before — it’s called the Affordable Care Act,” Durbin said, referring to the lengthy talks over health care reform under President Barack Obama. “We waited a calendar year for bipartisanship, and it never, ever appeared.”

The new proposal is the work of five Democrats and five Republicans: Sens. Romney, Bill Cassidy (R-La.), Susan Collins (R-Maine), Joe Manchin III (D-W.Va.), Lisa Murkowski (R-Alaska), Rob Portman (R-Ohio), Jeanne Shaheen (D-N.H.), Kyrsten Sinema (D-Ariz.), Jon Tester (D-Mont.) and Mark R. Warner (D-Va.).

Their early agreement calls for about $974 billion in infrastructure spending over five years, which would total about $1.2 trillion over eight years. More than half of the total amount constitutes new spending, less than the $2.2 trillion that Biden had put forward as part of his initial blueprint known as the American Jobs Plan this spring. Lawmakers said the plan includes no new tax increases, marking another break with the president, though it is expected to allow the gas tax to rise alongside inflation — and apply a version of it for the first time to electric vehicles — to help raise revenue.

The bipartisan bloc announced they had reached a deal Thursday, just three days after Biden said he would look to other avenues for compromise in the wake of collapsed talks with a group of Republicans led by Sen. Shelley Moore Capito (R-W.Va.). Biden said at the time that Capito’s package, which was smaller than lawmakers’ new bipartisan work product, did not go far enough or spend enough for his liking — leaving Capito skeptical that her colleagues are likely to fare much better.

“I want to see us have an infrastructure plan,” she told reporters, before noting that Biden has continued to seek some tax increases that Republicans do not support. “So I don’t know if they’re going to have any more luck.”

But early schisms emerged even before Senate lawmakers formally released full details of their proposal. For one thing, the proposal is expected to hew largely to a more traditional definition of infrastructure — a contrast with the White House’s plan, which seeks to target some spending at families and schools.

House Speaker Nancy Pelosi (D-Calif.) signaled on Sunday that her party is not willing to back down in pursuing those investments as part of either infrastructure reform or another future economic package.

“I have no intention of abandoning the rest of my vision,” Pelosi told CNN’s “State of the Union.” “What is being talked about in [infrastructure reform] is, by and large, something that could have been talked about 50 years ago. We’re talking about the future.”

Other Democrats, meanwhile, raised early fears that the bipartisan package might not go far enough on climate change, especially after the White House itself signaled an openness to rethinking its approach in pursuit of a deal with GOP lawmakers who disfavor the provisions. In recent days, Sen. Sheldon Whitehouse (D-R.I.) has lamented publicly there are “zero people” who see recent bipartisan talks “as a vehicle for anything serious on climate.”

Asked if their emerging infrastructure agreement includes proposals to address the issue, Tester said Thursday, “Oh, sure.”

Whether it is “enough,” he continued, “is the question.”

“There are some people that are going to say the climate provisions are more than enough, there are other people that are going to say it’s not adequate,” Tester said. “It’s going to be up to each individual member once this thing is truly done.”

Still more Democrats raised concerns with efforts to tie the gas tax to inflation: Even though it is not technically an increase, the proposal could still raise Americans’ costs at the pump, violating Biden’s pledge not to raise taxes on Americans making under $400,000. Inside the White House, top officials advising the president see the idea as a nonstarter, while some congressional Democrats said they shared that view.

The White House did not respond to a request for comment. Collins, one of the lawmakers behind the blueprint, sought to heed off potential criticism that it violates the president’s pledge during an interview Sunday.

“There won’t be a gas tax increase, and we won’t be undoing the 2017 tax reform bill,” she told CBS News’s “Face the Nation.”

The still-forming Senate proposal isn’t the only attempt at bipartisan compromise in the works: A bloc of House Democrats and Republicans, known as the Problem Solvers Caucus, unveiled their own eight-year, $1.2 trillion plan last week, about one-third of which constitutes new spending. The group’s leaders, Democratic Rep. Josh Gottheimer (N.J.) and Republican Rep. Brian Fitzpatrick (Pa.), shared their plan with top White House officials last week. They have yet to identify ways to finance the package.

“I know there are some who say we shouldn’t proceed in a bipartisan way,” Gottheimer said. He stressed that abandoning talks would be a “huge mistake,” adding, “I don’t know why we would walk away when we’re in the middle of it.”

But selling some of these retooled plans to Republicans could prove just as tough, particularly in the Senate, predicted Sen. Roy Blunt (R-Mo.), who participated in the earlier, unsuccessful round of talks.

“If we would have gotten a deal, I felt like there was a chance for 20 or 25 Republicans to be part of that deal,” he said. Blunt added it is “less likely among any sort of a gang-like effort” because of the fact that GOP leadership, including its committee chiefs, are less involved.

“A group of 10 that’s bipartisan only has five Republicans,” he continued, which he said presented two challenges. “One, you’ve got to get five more Republicans to get to 60. And two, if you can’t hold on to all 50 Democrats, you have to add” more Republicans.

In the meantime, Democrats in Congress have sought to keep their options open as they begin the broader process for setting government funding levels for the 2022 fiscal year. The Capitol’s two budget leaders — Rep. John Yarmuth (D-Ky.) in the House and Sen. Bernie Sanders (I-Vt.) in the Senate — are preparing reconciliation instructions that could advance Biden’s infrastructure proposal as well as his roughly $1.9 trillion families-focused plan. The move would allow Democrats to sidestep the GOP, but only if Democrats are in lockstep, and lawmakers including Manchin have said they aren’t yet ready to abandon bipartisan talks.

House and Senate lawmakers also have started moving a slew of additional bills that encompass many of Biden's infrastructure goals — some of which ultimately could become a vehicle for a broader spending package.

On Thursday, for example, the House took a key procedural step toward advancing a $547 billion bill to upgrade the country’s roads and fund other federal surface transportation programs. The bill shares many of the same contours as Biden’s jobs plan, including its inclusion of proposals to fight climate change, a move that prompted Republicans to vote overwhelmingly against it as it emerged from committee.

House Majority Leader Steny H. Hoyer (D-Md.) told lawmakers in a letter last week that Democrats intend to bring the proposal, chiefly written by Rep. Peter A. DeFazio (D-Ore.), to the floor before the end of June. The Senate, meanwhile, similarly has wrapped up early legislative work on its own roads-focused package — a more scaled-back, overwhelmingly bipartisan bill totaling about $300 billion. Lawmakers last week also introduced a $78 billion proposal accompanying it to improve the country’s railways. Congress must reauthorize many of these key transportation programs by the end of September, adding urgency to their efforts.

House and Senate lawmakers similarly have forged ahead in advancing tens of billions of dollars in bills to upgrade the country’s pipes, expand access to clean drinking water and fix aging wastewater facilities, much as Biden has sought to do. And the Senate last week adopted a roughly $250 billion bill to battle back China’s rise in science and technology that included new funds for chipmakers, another element of the president’s plan.

Seung Min Kim contributed to this story.