CNN  — 

Liberal Senate Democrats are unloading on a bipartisan group of senators working on an infrastructure deal, warning that the effort to win GOP backing could erode support on the left – underscoring the stiff resistance the negotiators face in shepherding the proposal through a narrowly divided Congress.

Even before 10 senators from both parties announced a broad agreement Thursday on an eight-year, $1.2 trillion plan, a number of members of the Senate Democratic Caucus threatened to oppose it, putting the effort in peril before the group has finalized many of the details.

The criticism has grown louder by the day, underscoring the growing tension within the ranks as moderates urge their colleagues to show patience and as Democratic leaders struggle to find a deal that can pass the 50-50 Senate and please the various factions within their party.

“Let’s face it. It’s time to move forward,” Sen. Elizabeth Warren, a Massachusetts Democrat, told CNN on the talks with the bipartisan group. “The Republicans have held us up long enough.”

Sen. Richard Blumenthal added: “I have no confidence that this bipartisan group will reach a deal. They should have a limited time to do so. I really think it’s time to pull the plug now and take action promptly and robustly … I worry about time being wasted.”

“We simply do not have the time to waste,” the Connecticut Democrat said.

The public rebuke is happening even as the White House and Senate Democratic leaders are giving negotiators from both parties time to see if they can cut a deal that can pass Congress, with a bipartisan group of 10 senators saying Thursday they had reached an agreement “on a comprehensive framework” and it would be “fully paid for and not include tax increases.”

The challenge for the White House and Democratic leaders – if they support this plan – will be to convince the left that the priorities left out of the bipartisan deal ultimately will be folded into the next proposal, which Democrats want to pass along party lines through the filibuster-proof budget process.

Of the $1.2 trillion in the bipartisan group’s proposal, $578 billion would amount to new spending. The cost over five years would be $947 billion, according to two sources familiar with the matter.

But the details still need to be written and face a tough task in winning enough support to become law.

Democratic leaders say they are pursuing Biden’s massive infrastructure and social safety net package along both bipartisan and partisan tracks. As the bipartisan talks continue, Senate Majority Leader Chuck Schumer is preparing to begin the budget process next month, setting the stage for advancing a bill along straight party lines, something that can only succeed if all 50 Democrats endorse such a process known as reconciliation.

“We are on two tracks: A bipartisan track and a reconciliation track, and both of them are moving forward,” Schumer told CNN Thursday.

Yet a number of Democrats say that whatever bipartisan deal moves forward is unlikely to win wide backing within their caucus.

“I think it’s been very clear to those negotiators, that we are rooting them on, but that there is no guarantee that you can get 50 Democratic votes for the package they produce,” Sen. Chris Murphy, a Democrat from Connecticut warned.

In particular, Democrats are raising concerns about how the package will be paid for – as Republican senators say that there would be no tax hikes and as Democrats have demanded new taxes on corporations and high-income earners to pay for the plan. But the bipartisan group is instead looking at redirecting already-enacted Covid-19 relief money, while raising the gas tax subject to inflation – ideas that a number of Democrats flatly oppose.

Asked about Republicans’ refusal to raise taxes to help pay for the plan, Hawaii Sen. Mazie Hirono said, “I totally disagree with” it.

The divisions underscore not just political differences within the Democratic caucus, but also regional ones. Many of the lawmakers in the bipartisan group hail from states outside of the Northeast corridor where residents rely heavily on rail and mass transit.

“I get worried when I see groups of senators that don’t include members from the Northeast corridor that really care about making sure that we dramatically change transit times,” Murphy said.

Sen. Bob Casey, a Pennsylvania Democrat, said that he’s “certain” that the bipartisan talks won’t produce “what I believe we have to do for the people of Pennsylvania,” calling for enacting both Biden’s $1.8 trillion American Families Plan and $2.3 trillion American Jobs Plan.

But Democrats have a problem: They don’t have consensus to pass such a massive bill along straight party lines, as moderates like Sens. Joe Manchin of West Virginia and Kyrsten Sinema of Arizona push to pursue bipartisan talks instead.

“Right now we don’t have the votes to do that,” Sen. Jeanne Shaheen, a New Hampshire Democrat who is also engaged in the bipartisan talks, said when asked if she’d back a Democrat-only approach through the reconciliation process.

“I say, let’s give it a little more time,” said Sen. Angus King, a Maine independent who caucuses with Democrats. “The legislative process was designed to be slow and cumbersome.”

With Biden traveling through Europe during a critical moment in his presidency, the White House has said the President will be amendable to phone calls while he’s abroad. Much of the input from the White House is expected to come from aides who stayed behind, including White House Chief of Staff Ron Klain and director of Legislative Affairs Louisa Terrell.

On the Republican side, members of the bipartisan group briefed Senate Minority Leader Mitch McConnell on Wednesday with GOP members telling reporters that McConnell signaled he was “open” to letting the talks play out.

“Mitch McConnell yesterday said he was open to it. That’s a good next step,” said Sen. Bill Cassidy, Republican from Louisiana, who is part of the group trying to hash out a bipartisan deal.

Other Republican leaders, however, have expressed doubt that any deal reached could garner the 10 Republicans needed to overcome a filibuster attempt.

Senate GOP Whip John Thune of South Dakota and Texas Sen. John Cornyn said that the amount of spending proposed would have to adhere close to what Republican negotiators already offered Biden – roughly $300 billion in new money and $1 trillion in overall spending – to garner widespread backing in the Republican conference. That number, however, was already rejected by the White House.

“I think he thinks he is going to get a better deal,” Cornyn said of Biden’s ongoing talks with a new set of Republicans. “But there is nothing that says whatever this group agrees to other Republicans are going to support. To me that is the flaw to this sort of approach.”

That talk has liberals concerned that 10 Republicans are unlikely to back any deal – even one that some of their members endorse.

“No,” Senate Budget Chairman Bernie Sanders, a Vermont independent who caucuses with the Democrats, said when asked if would support the bipartisan group’s potential agreement.

“In my view, now is the time to finally stand up for the working families of this country. Black and White, Latino, Native American, Asian American, that is what we have got to do,” Sanders said. “If your question is: Do I think there are 10 Republicans who are prepared to do that? No, I do not.”

This story and headline have been updated to reflect additional developments Thursday.

CNN’s Ted Barrett and Morgan Rimmer contributed to this report.