McClatchy: 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 …