A bipartisan group of centrist senators announced on Thursday that they had struck a tentative agreement on a framework for an infrastructure plan, as they mount a precarious bid to secure the blessing of the Biden administration, congressional leaders and rank-and-file lawmakers.

The statement, released by five Democrats and five Republicans, offered little detail about the substance of the agreement, beyond that it was “a realistic, compromise framework to modernize our nation’s infrastructure and energy technologies” that would be fully paid for without tax increases.

But the preliminary framework is expected to include about $579 billion in new spending, as part of an overall package that would cost about $974 billion over five years and about $1.2 trillion over eight years, according to two people familiar with the details, who disclosed them on condition of anonymity. The Biden administration had previously signaled support for a package that spent at least $1 trillion in new funds over eight years, on top of the expected maintenance of existing programs.

The announcement came after a dizzying day on Capitol Hill, where senators involved in the discussions offered conflicting assessments of their progress. It was unclear how quickly the group would unveil specific details or release legislative text, though members of the group said they were discussing the proposal with their colleagues and the White House.

The preliminary agreement, which sketches out funding for traditional physical infrastructure projects, faces steep headwinds, despite direct encouragement and personal outreach from President Biden. Lawmakers and aides in both parties remain skeptical that the group can muster the 60 votes needed to overcome a filibuster in the Senate, given deep divisions over funding levels and how to pay for it.