Sec. LAHOOD: The thing that I want you to know: There are a lot of people putting their heads together right now to figure out where to get $20 billion and how to pay for it.

CORNISH: Some senators want to use leftover stimulus funds to cover the shortfall, and there are all sorts of ideas for long-term funding, like raising the gasoline tax or taxing people based on the miles they drive. Either way, it would be a tax increase, so the president's 18-month plan would put that politically-unsavory debate safely aside until after midterm elections.

Audie Cornish, NPR News, the Capitol.