Washington ? Despite opposition from the White House, a proposal to tax motorists on the number of miles they drive each year is gathering speed on Capitol Hill.

Its popularity is increasing as Congress searches for alternatives to the federal gasoline tax, which isn’t indexed to inflation and hasn’t been raised since 1993.

Supporters say that a mileage tax would be a more reliable source of funding for the upkeep of the nation’s roads and bridges. Many environmentalists endorse it, saying that it would lead to less driving and less pollution.

However, the proposal is raising privacy concerns — particularly if GPS devices were to monitor mileage — and opponents say that the last thing people need is a new tax, particularly in the middle of a recession. Some critics, moreover, fear that it would have a disproportionate impact in states such as California, which has longer-than-average commutes.

A bipartisan commission that Congress created said last week that lawmakers should increase the gasoline tax by 10 cents per gallon but begin shifting to a mileage tax.

“With the expected shift to more fuel-efficient vehicles, it will be increasingly difficult to rely on the gas tax to raise the funds needed to improve — let alone maintain — our nation’s surface transportation infrastructure,” said Robert Atkinson, the chairman of the National Surface Transportation Infrastructure Financing Commission.