The US Senate holds a key test vote Wednesday on a $1.2 trillion infrastructure package, with the bipartisan deal at risk of failing at the first hurdle as Democrats and Republicans scramble to salvage a compromise.

Some four weeks after the president stood shoulder to shoulder with senators from both parties at the White House proclaiming a framework agreement had been reached to fix the nation's roads, bridges, ports, water pipes and internet connections, the deal remains under serious threat.

Senate Majority Leader Chuck Schumer dramatically raised the stakes last week, imposing a deadline of sorts by forcing an initial procedural vote that would allow debate to formally begin on the bill, even though haggling is still ongoing.

Advancing the measure, though, would need 60 votes, including at least 10 Republicans, in a chamber that is evenly divided 50-50.

But Republicans have threatened to torpedo it unless they are given more opportunity to work out the details, particularly over how to pay for the package, which includes $579 billion in new federal spending.

"We need more time," Senator Todd Young told AFP, saying he predicts Wednesday's procedural vote will fail.'

'Senator Mitt Romney, one of the 20-odd negotiators on the package, said he and his fellow Republicans are "not going to be voting for it today" because no deal has been finalized.

"But we'll have a pretty full outline, probably by Monday, and we'll be in favor of it then," Romney said.

Negotiations have stalled over revenue for the infrastructure package, with Republicans jettisoning a plan that would have raised billions by giving the Internal Revenue Service more power to combat tax dodging.

Republican hesitation, including over Schumer's tight time frame, has raised questions about whether the historic spending on infrastructure will ever get over the finish line.

It may have nudged Schumer to dial back the sense of a looming deadline.

"This week's vote is an honest attempt to get something done, to get the ball rolling," the Democrat told senators Tuesday.

"It's not a final deadline for legislative text," he stressed. "It is not a fish-or-cut-bait moment."

'It's not over'

The White House and the rest of Washington are watching closely.

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 once-in-a-generation investments in health, education, tackling climate change 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.

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

But the first step in Biden's sweeping agenda might have to wait. "The Vegas odds say we won't get enough Republican votes" Wednesday for the bipartisan infrastructure package, Senate Democrat Tim Kaine acknowledged.

"But it's not over," he told AFP. "If we don't get there today we should still keep at it."