Washington CNN  — 

Top Republicans have said they reached agreement on their party’s stimulus proposal and a formal rollout had been expected on Thursday, but as negotiators raced for a final deal the timing for the plan’s release has been delayed over disputes and holdups.

Treasury Secretary Steven Mnuchin emerged from Senate Majority Leader Mitch McConnell’s office on Thursday morning to announce a “fundamental agreement” between the White House and Senate Republicans on a $1 trillion coronavirus relief package. But Mnuchin and White House chief of staff Mark Meadows declined to commit to the public release of the plan Thursday. The proposal will now likely be released on Monday, a GOP aide told CNN.

That came as a surprise to rank-and-file GOP senators, who told reporters walking into their closed-door lunch on Thursday that the understanding had been that Republicans would release their multi-piece plan later in the day.

Instead, the top White House negotiators were left with touting “productive” discussions and said they were moving as “expeditiously” as possible.

The primary hold up, two sources said, was differences over the extension of the federal unemployment enhancement, specifically how Republicans would implement their changes to the $600-a-week program that expires July 31. Both sides are also still poring over the legislative text of agreed-upon pieces, which Meadows acknowledged is a time-consuming process.

McConnell said in remarks on the Senate floor late in the day Thursday that the Trump administration asked for more time to review the plan and that it should be out “early next week.”

“The administration has requested additional time to review the fine details, but we will be laying down this proposal early next week,” he said.

McConnell said that key GOP senators who have helped craft the package will introduce component pieces of the bill on Monday.

The delay in the release of the GOP proposal serves as an ominous signal given the scale of the challenge lawmakers and the White House are facing as they move to strike a bipartisan deal on a new stimulus package to get anything to the President’s desk before the end to federal unemployment enhancement used by millions of Americans.

The GOP stimulus plan will only serve as an opening bid ahead of what are sure to be hard-fought negotiations with Democrats.

Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer, who are unified behind a $3 trillion House-passed proposal of their own, have already panned the emerging GOP plan.

Pelosi, a California Democrat, said Thursday that “what we have seen so far falls very short of the challenge that we face in order to defeat the virus.”

Senate Democratic leader Chuck Schumer of New York said that Democrats are “waiting for the Republicans to put together a partisan bill that will never become law just so they can muster up the courage to negotiate.”

Finding a proposal that can unify Republicans has so far proven a challenge for the Senate GOP conference and the White House with days of public splits within the party on display over the proposal.

It is likely to be far harder to find common ground when bipartisan negotiations get underway, with the two sides trillions of dollars apart on the topline and diametrically opposed on several central components of their respective proposals.

Several GOP issues had been resolved by Thursday, however, including the White House dropping a push for a payroll tax cut and its insistence that new money for schools be tied solely to reopening, as well as a compromise over Senate Republicans’ push for new funds for state-based coronavirus testing.

The overall contours of the Republican have come into view and include $105 billion for education funding, $16 billion in new funds for testing, a second round of forgiveable small business loans, a second round of direct payments and series of tax incentives designed to help employers bring people back to work, and do so safely.

The real deadline negotiators are focused on, Mnuchin reiterated, is the expiration of the federal enhancement of unemployment insurance.

Mnuchin called discussions between White House negotiators and McConnell “very productive,” on Thursday but acknowledged that both sides still need to review and finalize text.

Meadows acknowledged the time crunch negotiators currently face, given the upcoming weekend and the expected funeral of the late Rep. John Lewis next week.

“We’re working expeditiously and it’s why we’ll probably be back up here this afternoon,” Meadows said.

Leaving a closed-door lunch earlier in the day on Thursday, McConnell was mum about the timing of the release of the GOP bill. “We’ll let you know,” he said.

Other Republican senators sounded uncertain over the timeline for the bill’s release on Thursday afternoon.

Indiana Republican Sen. Mike Braun said that someone asked the Republican leader during a closed-door lunch about the stimulus bill and McConnell’s message was that it was a “work in progress.”

This story has been updated with additional developments Thursday.

CNN’s Manu Raju, Lauren Fox and Ted Barrett contributed to this report.