The chamber kicked off what was already expected to be a marathon session set to include dozens of votes on amendments to the American Rescue Plan, which the White House has pressured Democrats to clear through Congress in the coming days.

The Senate's Democratic leader Chuck Schumer aimed to get the mammoth measure across the finish like this week, warning colleagues of several late-night votes and telling them: "we need to get this done."

But Schumer's effort was in peril when action on the Senate floor stalled for more than seven hours Friday amid negotiations over supplemental benefits for people unemployed during the coronavirus pandemic.

Moderate Senate Democrat Joe Manchin, keen on more fiscal restraint than was evident in the Biden package, was flexing his political muscle, reportedly balking at an amendment that would increase weekly additional unemployment benefits.

The current weekly benefits of $300 expire on March 14, heaping pressure on Congress to pass the plan within the next week.

In the bill that passed the House, the benefits were raised to $400 and extended through August 2021. 

But negotiations over a compromise were at an impasse, with Manchin apparently unconvinced about the alternative.

"We're stuck right now where we are," Republican Senator John Thune told reporters.

Meanwhile, Republicans offered their own alternative that they say would save $128 billion by reducing the benefits to $300 and extending them only to July.

West Virginia's Manchin, the most conservative Senate Democrat, is said to be weighing the dueling plans.

But Republican Lindsey Graham expressed frustration that Biden was pressuring Democrats to stay in line, rather than letting moderates work to craft a bipartisan solution.

"President Biden's inaugural speech (about bipartisanship) rings hollow," Graham said. 

"Instead of trying to bring us together, he's on the phone making sure a couple Democrats will not work with the Republicans. And that's so sad."

Democrats are in control of a Senate that is deadlocked 50-50, so any defections within the party could kill off Biden's package.

In the event of a tie, Vice-President Kamala Harris casts the deciding vote. 

The White House heaped pressure on Democrats Friday afternoon, tweeting that Biden is "urging the Senate to quickly pass the American Rescue Plan to provide Americans the direct relief they need."

Even before the paralysis over unemployment benefits, the process was shaping up to be an endurance test. 

Republicans on Thursday began by demanding that the 628-page bill be read aloud, in its entirety, on the Senate floor, a 10-hour recitation that only ended after midnight.