"Our view is we can't wait for some other funding mechanism to come out of the woodwork," Mortimer said. "We need to raise revenue this year, and in our view, the only one that is ready to go that we know works is the fuel tax."
Chris Spear, president and CEO of the American Trucking Association, said it's unclear why a known user fee -- the gas tax -- would be less politically feasible than the miles traveled tax, an untested fee with significant privacy concerns.
Spear said the trucking industry is still years away from a feasible program to charge drivers based on miles traveled.
It isn't clear that Republican members of the House would be any more likely to support a new tax on miles traveled than they would be to increase an old one on fuel.
Graves thinks they might. He said Wednesday that Speaker Paul Ryan, R-Wis., "likes the idea" of a miles traveled fee. Ryan's office did not immediately respond to request for comment.
Conservative groups that have lobbied hard against the gas tax hike signaled Wednesday that they aren't likely to support an entirely new tax in its place.
"Lawmakers can, and should, focus on smarter spending and breaking down burdensome regulatory barriers that delay projects and drive up costs, rather than turning back to hardworking taxpayers to further bear the cost of their own inefficient spending," said Nathan Nascimento, vice president of Freedom Partners, a conservative nonprofit.
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