After rejecting criticism that it is taking on too much, the Obama administration has identified one area where ambitious reforms will have to wait: overhauling the nation's aging, congested and carbon-emitting transportation system. The current six-year, $286 billion transportation spending plan expires in October, and House members have worked for months to produce a new 775-page, $500 billion bill that would create a new fund for road repairs, increase funding for rail and public transit and include reforms meant to wean the country from fossil fuels. But it became clear at a contentious Senate hearing Thursday that the half-trillion-dollar question is how to pay for the bill. The 18.4 cent federal gas tax has not been raised since 1993, and revenue from it falls increasingly short every year because of inflation and the shift to more fuel-efficient cars. Some in Congress, state governments and even in the freight-trucking industry believe the time has come to consider a greatly expanded revenue source, whether a higher gas tax or more tolls, or a shift to a 'vehicle miles traveled' fee, which drivers would pay based on how much they drive. The White House and some of its Senate allies are letting it be known, though, that this is not a discussion they want to have now, in the middle of a recession and as Washington is consumed with battles over health care and energy. Also, polls show Americans are growing anxious about government spending. Earlier this year, when they were criticized for not spending enough stimulus dollars for transportation, administration officials said their transformative plans for transportation would come in the six-year renewal this fall. But now the administration is telling Congress that it wants to extend the current spending plan by 18 months -- past the 2010 elections. That will require Congress to provide a $20 billion patch just to continue funding at current levels, because the highway trust fund supported by the gas tax is expected to run dry next month. The administration hopes it can attach some initial policy reforms that would benefit big metro areas and public transit. But the broader transformation should wait until later, officials say. 'President Obama does have a vision for transportation. It's not something he's going ignore or turn a blind eye to at all,' transportation secretary Ray LaHood told skeptical senators Thursday. 'The timing is where we part company.' Despite the White House call for delay, House Transportation and Infrastructure Committee Chairman James Oberstar, D-Minn., is pressing ahead with his bill. He and his House allies argue that states and cities need a new long-term bill to be able to plan future capital projects, and that the Washington Metro crash offered further proof that a badly frayed infrastructure needs serious attention now. Rep. Peter DeFazio, D-Ore., is proposing that if the White House and Senate will not consider a higher gas tax, then the bill could be paid for with a new tax on oil speculators. 'President Obama said to us during the campaign that we must have the fierce urgency of now. And that's what Mr. Oberstar has done,' said Rep. Elijah Cummings, D-Md. The debate flared Thursday at a hearing before the Senate Environment and Public Works Commmittee. Chairwoman Barbara Boxer, D-Calif., who is focused on passing a climate bill, said she favored the 18-month patch, and most members present agreed. But Sen. George Voinovich, R-Ohio, protested, saying a delay would duck the hard questions and that Americans would be willing to pay higher gas taxes for better transportation and new construction jobs. 'The way to get the job done is to pass a bill now. Urgent. Get it done,' he said. 'What the country needs to know is that in the next five year period we're going to make a comprehensive commitment to infrastructure, including high speed rail, highways, and the rest of it. ...Can you imagine what that would mean in terms of our economy and it giving people confidence in where we're going?' Boxer agreed but said a gas tax increase now was not feasible. 'I would tell you if you go out to the people of America and say that's the solution, I don't think they will buy it,' she said. 'They're struggling right now.' 