State transportation officials expressed alarm. The money shortage will have “grave repercussions for the states, for hundreds of thousands of workers in the construction industry and the driving public,” said John Horsley, executive director of the American Association of State Highway and Transportation Officials.

Contractors were also worried. “Each week that it goes on, it gets more serious,” said Brian P. Deery, senior director of the Associated General Contractors of America’s highway and transportation division. At some point, he said, some states may have to tell road contractors that they cannot pay them and that “we’d like you to continue working, but we understand if you have to stop working.”

In July, the House passed a bill that would use $8 billion of general federal revenue  from income and other taxes, not the dedicated motor fuel tax  to finance highway projects. The measure has not gained much support in the Senate, and until Friday the White House had been hostile to it. But Ms. Peters said the administration now endorsed the measure because “immediate action” was required to ensure that the states did not suffer.

Another possible solution would be to transfer money to the highway account from the account that the trust fund maintains to finance mass transit. But lawmakers from large cities that rely on trust-fund aid for their transit systems could be expected to resist such a move.

For the moment, Republicans and Democrats were blaming each other for the problem, which comes as the economy is in trouble and the election season is intensifying. The administration has accused the Democratic-controlled Congress of loading transportation bills with pork barrel projects that virtually invite President Bush’s veto.

But Democrats accuse the Bush administration of “nickel and diming our degrading roads, bridges, highways,” as well as mass transit, as Senator Charles E. Schumer of New York put it on Friday.

The issue is likely to be on a front-burner when Congress returns, as Representative James L. Oberstar, the Minnesota Democrat who heads the House Transportation and Infrastructure Committee, signaled on Friday. “Given that the United States has lost more than 600,000 jobs this year, and the unemployment rate is the highest it has been in five years, we cannot afford to lose one more family-wage construction job,” he said in a statement.

Whatever Congress does in the short term, some profound policy issues will have to be addressed at some point. The shift to smaller, more fuel-efficient cars in an era of expensive gasoline is the very trend that is helping to deplete the highway fund.