Democrats are growing anxious about a bipartisan deal reached last week by a group of moderates that is aimed at overhauling the nation’s infrastructure system.

The bipartisan framework, which totals approximately $1 trillion, deals only with physical infrastructure projects such as roads, bridges and waterways. Its costs would be offset in part by user fees on electric vehicles and indexing the gas tax to inflation. It would not include any tax increases on corporations.

Now, a growing number of Democratic senators are demanding some sort of assurance from leadership that Congress will pass another, more robust package ― one that includes progressive priorities ― if the senators first vote for the more narrow infrastructure bill negotiated with Republicans.

The fear is that some Senate Democrats ― Joe Manchin of West Virginia being just one of them ― would decline to support a robust second bill under a special budget process known as reconciliation, potentially dealing progressives a huge defeat.

“I’m not for this if it’s the beginning and the end,” Sen. Chris Murphy (D-Conn.)  when asked about the bipartisan infrastructure deal.

“We fail to do the second part of this at our political peril,” said Sen. Bob Casey (D-Pa.).

Democratic senators ranging from the progressive wing of the party to more centrist, rank-and-file members have expressed deep reservations about the agreement, which was negotiated by five Republicans and five Democrats. They are warning that investments to fight climate change, boost affordable housing and provide child care to millions of Americans can’t get left behind when all is said and done.

“No climate, no deal… You don’t get dessert until after you eat the main course,” Sen. Ed Markey (D-Mass.) said Tuesday, drawing the clearest red line about the possibility of a bipartisan infrastructure deal that would not include robust investments to fight climate change.

Sen. Richard Blumenthal (D-Conn.) called the spending level in the bipartisan infrastructure proposal “abysmally inadequate” to meet the needs of his state. He added that he would need an “ironclad guarantee” that Congress will do more in subsequent legislation before he could support it.

Asked what such a guarantee would look like, Sen. Tina Smith (D-Minn.) said it needed to be “chained together with a lock that cannot be broken.”

Other Democrats were more dubious about the prospect of trading their vote away in the present for a progressive bill that may never become law in the future. In all probability, the party would need total unanimity to approve such a bill, which would likely include things that moderate senators may have trouble supporting, like tax increases and at least an additional trillion dollars in spending.

“Put me down as skeptical of these theories that somehow you get everything you want, and somehow the priorities I have might be addressed down the road,” Sen. Ron Wyden (D-Ore.) told reporters on Tuesday.

Moderate Democrats like Manchin and Kyrsten Sinema of Arizona have not publicly affirmed their support for a reconciliation package. The two senators have expressed a desire to seek bipartisan consensus first.

For the moment, at least, Democratic leaders say they are proceeding with a two-track approach. They are allowing bipartisan negotiations with Republicans to continue even as they prepare to start the process of reconciliation, which would allow them to avoid a GOP filibuster.

Senate Majority Leader Chuck Schumer (D-N.Y.) plans to meet Wednesday with all Democratic members of the Senate budget committee to begin work on a budget resolution dealing with infrastructure. Schumer said he wants both the budget resolution and a bipartisan infrastructure deal to pass in July, setting up a huge month of action on Capitol Hill.

A bipartisan infrastructure deal could theoretically pass without the support of every Senate Democrat. But the math to get the 60 votes needed to avoid a filibuster gets dicey, considering more Republican votes would be needed to make up the difference. GOP leaders have not staked an official position on the bipartisan agreement, instead publicly taking a wait-and-see approach.

“We’ll see what happens,” Senate Minority Leader Mitch McConnell (R-Ky.) said Tuesday.

“In the end, it’ll be up to the majority leader to decide what to do,” he added.