That dampens proceeds from the federal tax on motor fuel: 18.4 cents a gallon on gasoline, 24.4 cents a gallon on diesel. Revenue from the tax goes to the Highway Trust Fund, with most of it designated to the highway account, which finances construction and repair of roads, and a much smaller share to the mass transit account.

The highway account had a balance of about $8 billion at the beginning of the fiscal year, and the expectation was that it would have some $4 billion by year’s end. But it has been emptying faster than anticipated, and the Bush administration now projects that at some point in the next fiscal year, beginning Oct. 1, it will hit zero, causing payments to be made from it only as revenue arrives. Ms. Peters predicted a shortfall of $3.1 billion.

Transferring the money from the transit account would require Congress’s approval, according to Congressional aides. And it is sure to face opposition.

“Robbing Peter to pay Paul is not the way to go,” said the president of the American Public Transportation Association, William W. Millar. “The administration proposal is shortsighted and would mean that the mass transit account would be reduced to the point where there would not be enough money to fund the federal transit program in 2010, even at the current level.”

Last week the House passed a bill that would spend $8 billion of general tax revenue on highways. But the White House said President Bush’s senior advisers would recommend that he veto any such provision, and called for borrowing from the mass transit account instead. That account is currently in surplus, the White House said, and the transfer “would not harm transit spending and would not increase the deficit.”