After a nearly 12 hour stall, the Democratic unemployment insurance agreement passed on a party line vote of 50-49.
The amendment, passed by Democrats in the early hours of Saturday morning, superseded an earlier Republican victory on unemployment insurance.
Democrats came to an agreement on unemployment benefits after initially hitting a snag early in the marathon voting session on the $1.9 trillion coronavirus relief legislation. Democrat Joe Manchin had threatened to unravel an agreement on how to handle jobless benefits in the package, but after eight hours of discussions, he agreed to a new proposal.
Democrats on Friday morning unveiled what they thought was an agreement on unemployment insurance, sponsored by Sen. Tom Carper, D-Del., that would cut the weekly jobless benefit from the $400 allotment in the House bill to $300, while allowing the benefit to continue through September rather than through August. The agreement also included the first $10,200 paid out through the unemployment program being untaxed.
But Manchin, who has been urging his colleagues and the White House to further target the bill, wasn't sold on the Carper proposal.
Manchin eventually agreed to a Democratic amendment to extend the enhanced UI program through Sept. 6 at $300 per week. The House-passed bill was through Aug. 29.
"The President supports the compromise agreement, and is grateful to all the Senators who worked so hard to reach this outcome," White House press secretary Jen Psaki said in a statement, of President Joe Biden's support. "It extends supplemental unemployment benefit into September, and helps the vast majority of unemployment insurance recipients avoid unanticipated tax bills. Most importantly, this agreement allows us to move forward on the urgently needed American Rescue Plan, with $1400 relief checks, funding we need to finish the vaccine rollout, open our schools, help those suffering from the pandemic, and more."
Senate Minority Whip John Thune, R-S.D., told reporters prior to the agreement he believed Democrats were working behind the scenes to keep their members united on some amendments.
"I just think that the Democrats right now are in a bit of a quandary," Thune said. "They've essentially stopped action on the floor so that they can try and persuade, I think, all their members to stay together on some of these votes. And I think they're afraid that they that they could lose on Portman."
The Senate is currently evenly split between Democrats and Republicans. To get Biden's signature piece of legislation passed into law, Democrats cannot afford to lose Manchin, or any other member of their caucus, on the overall vote. And if Manchin votes with Republicans on reducing the unemployment benefit, it risks upending support from progressives on the overall bill.
The balancing act has already required the administration to make other concessions.
Biden and Senate Democrats cut a deal Wednesday to lower the income threshold for who will receive partial direct payments. Individuals making under $75,000 and couples making under $150,000 will still receive a full direct payment, but partial payments will cap off at $80,000 and $160,000 respectively.
That deal appealed to Manchin and other moderate Democrats who hoped to see the direct payments given only to the most adversely impacted families and individuals.
If Democrats do manage to hang together, there is little Republicans can ultimately do to prevent the bill from passing. But that won't stop them from offering a laundry list of amendments to the bill in hopes of delaying a final vote.
The process could easily stretch into the morning hours of Saturday and beyond, depending on how motivated members are.
Minority Leader Mitch McConnell, who has repeatedly coined the relief bill a "liberal wish list" said Friday morning that his members have "many ideas to improve the bill."
"We are about to vote on all kinds of amendments in hopes that some of these ideas make it into the final product," McConnell said.
Already, the process was stalled for several hours because of a request from Sen. Ron Johnson, R-Wis., that the entirety of the 600-page bill be read allowed on the floor. It took over 10 hours to complete the process.
Before the amendment process commenced Friday morning, Senate Majority Leader Chuck Schumer set the stage for a long night but said the Senate will remain at it "no matter how long it takes."
The first amendment considered this afternoon came from Sen. Bernie Sanders, whose proposal would have raised the minimum wage to $15 an hour.
The House-passed version of the bill included the same proposal, but it was struck from the Senate bill after the senate parliamentarian ruled it out of bounds.
"Let’s be clear. This is the richest country in the history of the world," Sanders said. "We can no longer tolerate millions of our workers being unable to feed their families because they are working for starvation wages."
The Sanders amendment failed when eight Democrats joined with their Republican colleagues to kill the effort. Sanders said he'll continue fighting for a wage hike.
Many more amendments will be offered before the process concludes.
ABC News' Molly Nagle contributed to this report.