Washington CNN  — 

Several GOP senators who initially endorsed a bipartisan infrastructure deal are warning they may ultimately vote against it as it moves through the legislative process, a sign of the daunting hurdles ahead as proponents try to push the massive proposal through the evenly divided chamber by next month.

Initially, 11 Republicans signed off on an outline of the plan, which proposed to pump nearly $600 billion in new spending for “hard” infrastructure and cost a total of $1.2 trillion over the next eight years. Getting the bill through the Senate would require at least 10 Republicans to back the measure – assuming all 50 members of the Democratic caucus stay united, which is also highly uncertain.

But five of those 11 GOP senators told CNN on Monday that they are not committed to backing the bill, wary of some of the details for paying for the measure that have come to light and expressing misgivings about Democratic leaders’ plans to move the narrower bipartisan bill alongside a much-larger Democratic-only bill that would fulfill much of President Joe Biden’s economic agenda.

Sen. Jerry Moran, a Kansas Republican up for reelection next year, said he’s “concerned” by Speaker Nancy Pelosi’s plans to hold up House votes on a bipartisan deal until the Senate approves the Democratic-only bill, which could be approved by just 51 votes since it is expected to move through the filibuster-proof budget reconciliation process.

“I’m not there yet,” Moran said Monday evening. “I’m still interested in working to see if we can find a quality, bipartisan infrastructure plan. I want to be involved in that engagement and effort. But I’m still troubled by the statements of Speaker Pelosi. … It doesn’t seem the right kind of negotiating tactic to say, ‘Yeah, I’ll support a bipartisan plan, only as long as I get a vote on everything else I want.’ ”

The comments underscore the delicate task that Democratic leaders face to fulfill roughly $4 trillion of Biden’s agenda to make huge investments into the nation’s infrastructure and expand the social safety net. An influential contingent of moderate Democrats say they won’t back a large Democratic-only bill without trying to move a bipartisan bill first. Yet a vocal bloc of liberals refuse to back a narrow bipartisan bill without a commitment that the larger bill will be enacted.

So Pelosi drew a firm line late last month: “There ain’t going to be no bipartisan bill, unless we have a reconciliation bill.”

Senate Minority Leader Mitch McConnell, who has yet to take a position on the bipartisan deal, has been sharply critical of the Democrats’ tactics. And Republicans believe that if he urges his colleagues to vote against the bipartisan deal, it could ultimately collapse.

“What Speaker Pelosi does I can’t control, but they may not get anything if they start putting in conditions,” said Sen. Mike Rounds, a South Dakota Republican who also endorsed the bipartisan framework.

Of the 11 Republican senators who announced they were supporting a bipartisan infrastructure framework in June, a number say they have serious concerns about whether their proposal could be held hostage until Democrats complete their own party-line bill that will include massive social policy revisions on child care and sick leave and an extension of the boosted child tax credit. That bill is expected to make sweeping changes to the tax code on the personal and business side and raise the corporate tax rate.

Republicans have said tying the two proposals together was not part of the agreement and that the added comments from Democrats like Pelosi in the House and even the President have not been helpful to try to sell the proposal to their GOP colleagues.

“Things are in flux,” Sen. Todd Young, a Republican from Indiana who faces voters next year and is one of the 11 GOP backers, said Monday. “This is where the President needs to show leadership.”

Beyond the process, several of the GOP senators who backed the initial outline said Monday that there are real concerns about how the bill would be paid for – including a controversial provision to bolster tax collection enforcement by the IRS.

“No. No, no, no. I’m not committed or totally guaranteed on it,” Rounds said. “I want to support it, but I’m going to wait and see the final product as well, but I’m hoping that it works.”

Added Moran: “I think there needs to be constraints on what the use of those IRS agents would be. So we’re taking a look at that. That pay-for has some red flags among Republicans.”

Sen. Richard Burr, a North Carolina Republican who signed on to the framework, made clear he needs to learn more when the language is finalized and the official estimate from the nonpartisan Congressional Budget Office is released before he will commit to voting for it.

“Pay-fors are always a function of a little fuzzy math. So we’ll see,” the retiring GOP senator said Monday.

Burr added: “We haven’t negotiated the final language. Until that gets negotiated, there is no deal yet.”

GOP Sen. Lindsey Graham of South Carolina, who also signed off on the plan but has raised concerns about the Democrats’ tactics, said: “We don’t have enough pay-fors” to finance the full package.

‘There are lot of issues that are outstanding’

After two weeks away during the July Fourth recess, the bipartisan group is expected to meet again Tuesday, which members and aides say will be an opportunity to try to iron out key details of the bill and give them a chance to attempt to come back together after the group’s unity was tested in recent weeks.

“We’re going to have to spend the time and energy to deal with all the issues outstanding, and there are a lot of issues that are outstanding,” said Sen. Mitt Romney, a Utah Republican who’s one of the leaders of the bipartisan effort. “This is a major piece of legislation, which deals with the work of many committees, and so I can’t possibly predict how long it will take.”

Over the recess, staff worked to try to turn the bipartisan group’s framework into legislative text, but that work still isn’t completed, lawmakers say.

Some of the bipartisan group’s ideas to pay for the investments in roads and bridges – like public-private partnerships – have been questioned as being real revenue generators. The group’s idea of giving the IRS additional money to go after people who don’t pay what they owe in taxes has been criticized by outside conservative groups. The fear among some Republicans is that if the bipartisan proposal costs too much and isn’t offset enough, it will hemorrhage GOP votes.

Some of McConnell’s top deputies have sounded dour about the proposal.

“My concern is we borrowed a lot of money during the Covid crisis and we shouldn’t borrow any more for non-emergency items to pay for it,” said Sen. John Cornyn, a Texas Republican who’s a member of McConnell’s leadership team.

Asked about the vow by the bipartisan group to fully fund the plan, including by redirecting Covid-19 relief money, Cornyn said: “There’s a big hole to fill, and what I’ve seen so far doesn’t indicate they’ve filled it.”

Sen. John Thune, a South Dakota Republican who’s the minority whip, said how the measure is financed is ultimately going to be the key factor.

“There are still a lot of outstanding questions,” said Thune, referencing efforts to find cost offsets for the bill. “The details on this matter.”

CNN’s Morgan Rimmer and Ali Main contributed to this report.