Sen. Susan Collins, R-Maine, arrives for a meeting at the Capitol on Wednesday. Congressional negotiators and the White House appear open to striking a roughly $1 trillion deal on infrastructure, but are struggling with the hard part – deciding who will pay for it. Associated Press/Alex Brandon

WASHINGTON — A bipartisan group of senators tentatively reached an agreement with White House officials Wednesday on hundreds of billions of dollars in new spending for the nation’s infrastructure system, giving a significant boost toward one of President Biden’s biggest domestic policy ambitions.

Multiple senators leaving an evening negotiating session at the Capitol said the group – which included five Democrats, five Republicans and top White House officials – had reached a framework of a deal. They said senators would go to the White House on Thursday to brief Biden personally on the details.

“There’s a framework of agreement on a bipartisan infrastructure package,” Sen. Susan Collins, R-Maine, said, exiting the meeting. “There’s still details to be worked out.”

Senators declined to disclose details of their agreement but stressed that the group had agreed not just on the spending levels for various infrastructure projects, but also how to pay for it. An earlier framework reached by the senators – which did not have White House approval – included $974 billion of spending over five years, including $579 billion in new projects and initiatives.

“White House senior staff had two productive meetings today with the bipartisan group of Senators who have been negotiating about infrastructure,” White House press secretary Jen Psaki said in a statement. “The group made progress toward an outline of a potential agreement, and the President has invited the group to come to the White House tomorrow to discuss this in-person.”

Other tasks that remain for the bipartisan group of Senate negotiators include briefing their leadership and their respective caucuses. But, Collins said, “I’m optimistic that we’ve had a breakthrough.”

White House officials huddled with Democratic leaders immediately after the negotiations concluded Wednesday night on Capitol Hill, a sign they were already plotting next steps. Senate Majority Leader Charles Schumer, D-N.Y., said “we support the concepts that we heard about” but deferred further comment until the agreement was officially announced. It is less clear whether GOP leaders will sign off on the package, though White House officials have long felt it would be politically treacherous for Republican leaders to try to oppose a bipartisan infrastructure package that would improve highways, roads and bridges across the United States while creating large numbers of jobs.

The 10 senators – Collins; Bill Cassidy, R-La.; Joe Manchin III, D-W.Va.; Lisa Murkowski, R-Alaska; Rob Portman, R-Ohio; Mitt Romney, R-Utah; Jeanne Shaheen, D-N.H.; Kyrsten Sinema, D-Ariz.; Jon Tester, D-Mont.; and Mark Warner, D-Va. – have met numerous times with the administration’s negotiators this week. Steve Ricchetti, counselor to the president, legislative affairs director Louisa Terrell and National Economic Council director Brian Deese have been representing the administration in talks.

Asked about the ongoing negotiations at the White House earlier Wednesday, Biden said, “I’ll tell you that when I get the final numbers tonight.”

Though the agreement could amount to one of the most significant investments in infrastructure in recent years, the tentative bipartisan deal still falls far short of Biden’s initial, $2.2 trillion vision that he outlined in the so-called American Jobs Plan earlier this year. In that plan, Biden proposed paying for new spending by raising the corporate tax rate from 21 percent to 28 percent. The White House has also released a separate, $1.8 trillion package meant to invest in “human infrastructure” such as education and paid leave.

Many Republicans objected to the idea of raising the corporate tax rate, however, and White House officials have looked for other alternatives that would help them offset the new costs.

Democratic leaders have long said Biden’s expansive infrastructure agenda could pass through multiple tracks, meaning one bipartisan package that spends money on traditional public-works projects and another one that funds Democratic priorities that could pass using a special budgetary process that short-circuits a filibuster in the Senate.

To that end, House Speaker Nancy Pelosi, D-Calif., and Schumer met with administration officials on Capitol Hill Wednesday night to begin mapping out both legislative strategies for the president’s infrastructure plan.

Behind the scenes, Senate negotiators and administration officials tussled primarily over how to pay for the package. Unraveling the 2017 GOP tax law, which dramatically lowered the tax rate for corporations, was a nonstarter for Republicans. The bipartisan group initially proposed indexing the gas tax to inflation to raise additional revenue, but the White House said doing so was off-limits to them.

Details were not clear Wednesday night on how exactly the group plans to pay for its package. More information was expected Thursday after the senators brief Biden at the White House.

Manchin said the broad contours of the spending levels in the package had not changed.

“Wes owe it to the president to bring him up to speed on everything and they run the numbers too to make sure all the numbers we have are accurate,” he said.