President Joe Biden on Thursday endorsed a framework for legislation aiming to overhaul the nations infrastructure system, adding momentum to a bipartisan agreement that was cobbled together after weeks of painstaking negotiations by lawmakers.

We have a deal, Biden announced at the White House after a meeting with senators.

The framework totals roughly $1.2 trillion over eight years, containing $579 billion in new federal spending on narrowly defined infrastructure projects such as roads, bridges, airports and waterways. Its cost would be offset by a variety of revenue sources, but would not include any tax hikes.

We have agreed on the price tag, the scope, and how to pay for it, Sen. Susan Collins (R-Maine) said. It was essential to show the Senate can function that we can work in a bipartisan way.

The agreement was negotiated by a group of 10 senators, five Republicans and five Democrats, with heavy input from the White House. Twenty senators now support the agreement, a sizable number but still far short of the 60 votes required to enact it into law.

Lawmakers say some details still need to be ironed out as they shift to putting numbers to paper and turning out actual legislative text, including the most sensitive part of the proposal how it is to be paid for.

We havent written this down yet, and in terms of legislation, theres going to be a lot that happens down the road, Sen. Mitt Romney (R-Utah) told reporters on Wednesday.

The biggest challenge for Biden and other Democratic leaders will be selling the infrastructure package to their skeptical rank-and-file members. Progressives, in particular, have voiced loud concerns with the bipartisan framework namely, that it doesnt include robust measures to fight the increasing threat of climate change. They also want some kind of commitment that, if Congress passes the narrower bipartisan deal, it will also pass a broader jobs package that includes progressive priorities like child care, elder care, affordable housing and investments aimed at boosting electric vehicles.

Biden said he would insist on pairing the infrastructure deal with the broader package.

If this is the only thing that comes to me, Im not signing it, Biden said at a press conference. Im not just signing the bipartisan bill and forgetting the rest of it.

The fear progressives have is that some moderate Democrats, like Sen. Joe Manchin of West Virginia, will decline to support a robust second bill under a special budget process known as reconciliation that requires all 50 Democratic votes potentially dealing progressives a huge defeat.

In a Thursday interview on CNN, Senate Majority Whip Dick Durbin (D-Ill.) said its a key question whether all Democrats would support the bipartisan infrastructure agreement.

We also want to make sure all Democrats are on board for reconciliation, Durbin added.

One cant be done without the other, Senate Majority Leader Chuck Schumer (D-N.Y.) stressed Wednesday after a meeting with House Speaker Nancy Pelosi (D-Calif.).

There is a theory that agreeing to a narrow trillion-dollar infrastructure deal could make the tricky reconciliation process easier for Democrats. The $6 trillion reconciliation package Democrats are aiming to draft next month would thus be a trillion dollars less costly, making it easier for moderate Democrats to support.

But all that would depend on the whims of lawmakers like Manchin and Sen. Kyrsten Sinema (D-Ariz.), who have given no indication of locking themselves into supporting any legislation upfront, and who are unlikely to vote for another hugely expensive package.

We have to see whats in the other plan before I can say, Oh yes, you vote for this, and Ill vote for that. Thats not what I have signed up for, Manchin told reporters on Thursday.

However, other Democrats , such as Sens. Michael Bennet (Colo.) and Elizabeth Warren (Mass.), are echoing party leaders who say the two bills have to go together. Warren even insisted that the pair should be described as a single package.

It may be multiple votes, but its one deal, Warren told HuffPost.

Progressive priorities omitted from the bipartisan infrastructure package include tax hikes on the rich, billions for child care and a continuation of the monthly child tax credit payments that start in July.

A key player in the process, Senate Budget Committee chair Sen. Bernie Sanders (I-Vt.), was noncommittal about the infrastructure deal on Thursday. The negotiators have released few details.

Why dont you tell me whats in it someday and Ill tell you if I support it, Sanders told a reporter.

The other key player to watch in the process is Senate Minority Leader Mitch McConnell (R-Ky.), who has taken a hands-off approach to the bipartisan framework, declining either to attack or endorse it outright.

McConnells actions could determine the fate of the deal, given his sway over the GOP conference. The more Republicans who endorse the proposal, for example, the fewer Democratic votes would be needed to enact it into law.

Pelosi, meanwhile, has her own issues to contend with a majority in the House with a margin of just a handful of seats, and a bloc of progressives who have warned leaders not to allow their priorities to be left behind.

The California Democrat has vowed to delay a vote on a bipartisan infrastructure deal in the House until the Senate passes a robust reconciliation package. The move would maximize her leverage, but could potentially create a messy standoff with Senate Democrats.

We will not take up a bill in the House until the Senate passes the bipartisan bill and a reconciliation bill, Pelosi said Thursday. If there is no bipartisan bill, then well just go when the Senate passes a reconciliation bill. But Im hopeful that we will have the bipartisan bill.

Democrats want to pass both the bipartisan infrastructure deal and instructions for a reconciliation package in the coming weeks, setting up what is expected to be a busy and chaotic July on Capitol Hill.

This story has been updated with remarks from Biden, Warren and Sanders.