The US Senate stumbled Wednesday in the first test for a long-sought bipartisan infrastructure package as Republicans blocked the trillion-dollar measure's advancement, but lawmakers suggested a compromise deal was on the horizon.

With Republican senators in the 50-50 chamber uniting to oppose the package moving forward in a key procedural vote, Democrats were well short of the 60 votes needed to overcome a filibuster and formally begin debate on the plan.

Republicans argue that more time is needed to thrash out the details with Democrats and the White House, primarily how to raise sufficient revenue to fund the once-in-a-generation spending.

But Republicans and Democrats alike expressed optimism that a deal was days away.

"We'll have a pretty full outline, probably by Monday, and we'll be in favor of it then," Republican Senator Mitt Romney told AFP.

Romney and the other 21 negotiators issued a joint statement after the vote saying they have made "significant progress and are close to a final agreement."

"We will continue working hard to ensure we get this critical legislation right -- and are optimistic that we will finalize, and be prepared to advance, this historic bipartisan proposal... in the coming days."

President Joe Biden has made improving America's roads, bridges, ports, water pipes and broadband internet a priority in his sprawling domestic agenda.

Last month he stood with senators from both parties at the White House proclaiming a framework agreement had been secured, but after weeks of haggling a deal remained out of reach.

Traveling to Cincinnati, Ohio on Wednesday for a CNN town hall interview, Biden made a point of nudging Republicans on their informal agreement, reminding them that "I come from a tradition in the Senate, you shake your hand, that's it. You keep your word."

When asked whether he believed the plan would move forward in the Senate on Monday, Biden replied with an unequivocal "I do."

Senate Majority Leader Chuck Schumer had dramatically raised the stakes for the package by forcing Wednesday's procedural vote.

Republicans expressed frustration with the arbitrary deadline, but Schumer and Biden are keen to secure at least one major legislative victory before Congress breaks for its August recess.

Multiple issues are at play but the biggest sticking point remains how to pay for the package, which includes $579 billion in new federal spending.

Republicans recently jettisoned a plan that would have raised billions by giving the Internal Revenue Service more power to combat tax dodging.

Wednesday's vote was 49-51, with Schumer using a procedural tactic of changing his vote from yes to no so he can bring the bill up at a future time.

'Russian roulette'

The infrastructure package is a primary element of Biden's sweeping domestic agenda aimed at transforming America with more than $4 trillion in federal spending.

Democrats have announced they will move on the bulk of that plan with a go-it-alone, $3.5 trillion budget framework that includes massive investments in health, education, climate change resilience and expanding social welfare programs.

Democratic leaders intend to use a fast-track process known as reconciliation that allows budget-related legislation to pass by simple majority.

With Republicans united against the broader budget bill, every Senate Democrat would need to support the package -- no guarantee in a caucus that includes progressives and moderates.

If all Democrats back it and every Republican opposes, Vice President Kamala Harris, in her role as president of the Senate, would break the 50-50 tie.

"We don't have much leeway," progressive Senator Bernie Sanders told reporters.

"But I believe that at the end of the day, every member of the Democratic caucus understands that we have got to address the long-neglected needs of working families and that we need to bring forward this very, very comprehensive piece of legislation."

Republicans have expressed alarm at Biden's agenda, declaring it an attempt at a socialist takeover that would threaten the country's financial stability and fuel inflation.

"They're playing Russian roulette with our economy," top Republican Senator Mitch McConnell said.

Biden himself has pressed Congress to deliver on his priorities, saying last Friday that "we can't afford not to make these investments."