WASHINGTON (AP) — A $1 trillion bipartisan infrastructure deal senators struck with President Joe Biden is at risk of stalling out as Republicans mount stiff resistance over ways to pay for it and momentum shifts to a more robust Democratic proposal coming into focus Tuesday.

Biden’s big infrastructure proposals are moving on parallel tracks in Congress in a race against time and political headwinds to make a once-in-a-generation investment in the nation. Senators from both groups are huddling privately again late Tuesday evening to shore up their proposals. But the bipartisan deal is running into opposition from business leaders, outside activists and GOP senators potentially denying it the support that’s needed for passage.

Senate Majority Leader Chuck Schumer told Democratic colleagues to remain united — “Don't draw any lines in the sand,” he said — as they draft the bigger, multitrillion-dollar package of once-in-a-generation investments for the nation that are the top priority for the president and his party. As Democrats push ahead, Republican leadership said it's unlikely that the smaller, bipartisan effort would be ready for a Senate vote next week, as hoped.

“I’m going to take this day by day and participate in the process and see where we end up,” said Sen. Jerry Moran, R-Kan., who is part of the bipartisan group of 21 senators but is not fully committed to the plan.

Paying for the new infrastructure was always going to be a challenge, which is partly why public works investments have lagged over time. Biden has proposed raising taxes on corporations and wealthy Americans earning more than $400,000 a year, which would cover not only the nearly $1 trillion proposal, but also the broader Democratic plan that is now swelling beyond $3.5 trillion. Republicans reject that approach.

Instead, the bipartisan group of senators is racing to salvage its plan, straining to come up with other revenue streams to fund the $1 trillion package, which includes about $579 billion in new spending beyond regular expenditures that are funded by gas taxes and other sources.

One proposal to go after taxpayers who skip out on income taxes initially had potential bipartisan appeal, but now is being lambasted by the outside groups as a way to enable the IRS to snoop around Americans' personal finances. It would boost the IRS by $40 billion to bolster staff to audit tax returns, unleashing as much as a $100 billion net increase in revenues to federal coffers.

Sen. Kevin Cramer, R-N.D., said funding the IRS to audit potential tax scofflaws "is just way too undefined and nebulous and frankly eerie-sounding to most Republicans to be serious, in my view.”

Another proposal calls for reinstating fees that chemical companies used to pay for cleaning up the nation’s worst hazardous waste sites, which could bring in about $13 billion over 10 years. Those fees were allowed to expire in 1995, and the cleanup efforts are funded by general revenues. Biden has called for restoring the fees “so that polluting industries help fairly cover the cost of cleanups.”

But the American Chemistry Council called on lawmakers to remove the fees, saying they would likely be paid by consumers in the form of higher costs.

Money could come from $125 billion in COVID-19 relief funds approved in 2020 but not yet spent, as well as untapped unemployment insurance funds, among a hodgepodge of other sources.

Ed Mortimer of the U.S. Chamber of Commerce said some of the group’s members have concerns about some revenue sources in the bipartisan framework, but he added, “This is an investment we believe is worth making.”

As the bipartisan group struggles to devise consensus over revenue streams, it’s becoming increasingly clear that their proposal might not be fully paid for at all, making the outcome of the deal that was struck with much fanfare last month at the White House uncertain.

Senate Minority Leader Mitch McConnell, R-Ky., said he remained hopeful the bipartisan effort could proceed. But his earlier insistence that it "ought to be credibly paid for” signaled the party’s stance.

Ten Republican senators would be needed to back the bipartisan bill, joining with all 50 Democrats to reach the 60-vote threshold typically required to overcome a filibuster and advance it toward passage.

Meanwhile, the broader Democratic framework being compiled by independent Sen. Bernie Sanders and senators on the Senate Budget Committee he chairs is swelling past $3.5 trillion and gaining momentum.

Behind closed doors at the senators' lunch Tuesday, Sanders made the case for why Democrats should go big. According to a person who spoke on condition of anonymity to discuss the private meeting, Sanders encouraged his colleagues to focus primarily on the needs of America's working people and the climate crisis, rather than a topline budget number.

Sanders met with Biden at the White House on Monday and said they are on the same page in seeking a “transformative” investment for the nation.

Once rivals for the White House, Sanders and Biden are now joining forces to shape the president’s top priority.

“My job is to do everything I can to see that the Senate comes forward with the strongest possible legislation to protect the needs of the working families of this country,” Sanders said.

“The end of the day, we’re going to accomplish something very significant,” he said.

The emerging Biden plan would be the most substantial investment the nation has seen in years, beyond highways and broadband into the routine services of everyday American life — on par, the president and Sanders have said, with the New Deal of the 1930s.

There are funds to build child care centers and help families pay for that care, and expanded health care options for older Americans including dental and vision benefits. Public works would be bolstered to remove lead in drinking water pipes, enhance electric vehicle markets and rebuild communities to try to make them resilient to harsh weather and climate change.

A broad range of centrist and progressive senators seems to agree with the push for an expansive package, exiting a meeting with White House advisers late Monday at the Capitol and saying they want to produce a bold plan that “meets the moment,” as Sen. Debbie Stabenow, D-Mich., put it.

Under budget rules, Democrats could pass the proposal on their own in the evenly split Senate, without the 60 votes typically required.

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Associated Press writer Alan Fram in Washington contributed to this report.