Washington — After an early morning breakthrough in talks, the Senate hoped to vote Wednesday on a $2 trillion stimulus bill responding to the coronavirus pandemic, but a standoff emerged late in the day that threatened to delay its speedy passage.

The legislation is the most expensive stimulus package in U.S. history. Congressional aides were finalizing the text of the bill throughout the day as lawmakers circulated summaries of its provisions, including an expansion of unemployment benefits, direct payments of $1,200 to most individuals, and hundreds of billions of dollars for companies, hospitals and state and local governments.

The expansion of unemployment benefits became a point of contention that could potentially derail efforts to quickly bring the measure to the Senate floor for final passage.

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The bill increases the maximum unemployment insurance payment by $600 per week and extends the period for which workers are eligible to four months. Four Republican senators — Senators Tim Scott, Rick Scott, Ben Sasse and Lindsey Graham — raised concerns about what they called a "drafting error" in the bill, arguing that the expansion could incentivize workers to lose their jobs if the totality of state and federal unemployment benefits exceeds their current income.

The GOP senators proposed capping total unemployment insurance at 100% of income, and said they were preparing an amendment to that effect. That prompted Senator Bernie Sanders to threaten to place a "hold" on the bill unless Scott, Sasse and Graham dropped their objections.

"In my view, it would be an outrage to prevent working-class Americans to receive the emergency unemployment assistance included in this legislation. Unless these Republican senators drop their objections, I am prepared to put a hold on this bill until stronger conditions are imposed on the $500 billion corporate welfare fund," Sanders said in a statement posted to Twitter.

Senator Lindsey Graham speaks while flanked by Senators Tim Scott, Ben Sasse and Rick Scott during a press conference at the Capitol on March 25, 2020. Getty

However, the communications director for Senator Chuck Grassley, who spearheaded this part of the bill, said there was not a "drafting error" regarding unemployment insurance. Although receiving an additional $600 would leave unemployed workers more than their typical salary in some states, but there wasn't enough time to come up with a different formula for every state.

"This wasn't a drafting error. Nothing in this bill incentivizes businesses to lay off employees, in fact it's just the opposite," said Taylor Foy, Grassley's communications director. "Each state has a different UI program, so the drafters opted for a temporary across-the-board UI boost of $600 dollars, which can deliver needed aid in a timely manner rather than burning time to create a different administrative regime for each state. This increase is designed to make the average worker whole. It's also important to remember that nobody who voluntarily leaves an available job is eligible for UI."

A top GOP aide told CBS News that he believes the group of Republican senators who are oppose this this will be mollified if they get the chance to vote on a separate amendment that would prevent unemployed Americans from getting more money from insurance than they do on the job.

Treasury Secretary Steven Mnuchin said Wednesday evening that he did not believe the extra $600 would be an incentive for people to leave their jobs. He also said that he expected the bill to pass in the Senate later on Wednesday.

"Our expectation is, this bill passes tonight and gets to the House tomorrow," Mnuchin said.

The dispute came hours after leaders of both parties heralded the agreement on the package, forged after lengthy talks.

"After 5 days of arduous negotiations, after sleep deprived nights and marathon negotiating sessions, we have a bipartisan agreement on the largest rescue package in American history," Senate Minority Leader Chuck Schumer said in a floor speech shortly before 2 a.m.

Schumer and other congressional Democrats had argued that the original bill proposed by Senate Majority Leader Mitch McConnell last week did more to aid corporations than working Americans and failed to establish oversight for a loans to large corporations. Democrats blocked procedural votes on the McConnell legislation on Sunday and Monday, while Schumer negotiated with Treasury Mnuchin and other White House officials to craft a deal.

"Like all compromises, this bill is far from perfect. But we believe the legislation has improved sufficiently to warrant its quick consideration and passage," Schumer said Wednesday afternoon. "Democrats are ready to speed up consideration of the bill as much as possible."

In his own floor speech, McConnell said it was necessary to vote for the "emergency relief" package as soon as possible, saying the Senate had "stepped up" to the task.

"This strange new reality has forced our nation onto something like a wartime footing," McConnell said. "A fight has arrived on our shores. We did not seek it. We did not want it. But now, we are going to win it."

Senate Majority Leader Mitch McConnell, Secretary of Treasury Steven Mnuchin and Minority Leader Chuck Schumer hold a meeting to discuss a potential economic bill in response to the coronavirus in Washington on March 20, 2019. Getty

The Senate is expected to go into recess until April 20 after approving the bill.

If approved by the Senate, the measure would go to the House for a vote. Speaker Nancy Pelosi, who introduced her own competing bill earlier in the week, indicated in a statement that she was pleased with the final Senate package but stopped short of committing to bringing it for a vote as-is.

"The Republican bill proposed by Senator McConnell on Sunday was a non-starter. This bipartisan legislation takes us a long way down the road in meeting the needs of the American people," Pelosi said. "House Democrats will now review the final provisions and legislative text of the agreement to determine a course of action."

Pelosi ruled out the possibility of House approval by unanimous consent Wednesday evening. She said "that's not going to happen," and added, "Republicans have told us that's not possible from their side."

She also said that because of the magnitude of the bill, she'd like "to see a good debate on the floor," and then a voice vote.

If there is a voice vote, whoever is presiding over the floor will call for members to voice "yea" or "nay" and determine which side prevails. That would allow any member who wants to travel back to Washington to do so, but does not require everyone to travel back for a recorded vote. However, a single member could derail the vote in the House by calling for a recorded vote. In that case, Pelosi said "if you couldn't come, you could vote by proxy."

House members would also be given at least 24 hours' notice to return to Washington, Pelosi said.

President Trump said during a briefing by the Coronavirus Task Force that he would sign the legislation if passed.

"I encourage the House to pass this essential legislation and to send it to my desk without delay. I will sign it immediately," Mr. Trump said.

Mr. Trump said that he hoped the bill would aid the economy for "hopefully a long time."

The precise timing of the Senate vote remained in limbo, given the dispute over benefits.

The newly negotiated package provides $150 billion to invest in hospitals, and boosts funding for state, local and tribal governments to respond to the crisis. It prohibits businesses controlled by the president, vice president, members of Congress and heads of executive branch agencies from receiving loans or investments from Treasury programs. It would also require any loans granted to big businesses to be disclosed, a key sticking point in earlier talks.

The package is the third phase of the congressional response to the coronavirus pandemic. The president has previously signed an $8.3 billion emergency relief bill and another response bill written by House Democrats which mandated free testing and paid medical and family leave for certain workers.

Nancy Cordes, Bryce Klehm and Rebecca Kaplan contributed to this report.