WASHINGTON - For months and months, Washington politicians and policy makers have been casting about for a replacement for the fuel tax.

With a steady decline in revenue produced by the current federal taxes on gasoline and diesel fuel, the search has been on for a new or different source of revenue to support the nations highway system.

One alternative that has gained considerable interest is a vehicle miles traveled tax.

Now, however, many lawmakers, U.S. Department of Transportation officials and other experts are expressing concern with elements of the proposed tax. They also caution that a transition to a vehicle miles tax could be a slow process.

Roy Kienitz, a USDOT undersecretary for policy, said during a Senate Environment and Public Works Committee session last month that any solution for overcoming sliding revenue from the fuel tax would be "technically difficult and politically difficult."

Kienitz was pressed by Sen. Tom Carper, D-Del., about what USDOT was doing to prepare for a transition to a new sustainable funding stream.

"It is a matter on which weve given a fair amount of thought," Kienitz said, but he added that while USDOT was "working really hard to prepare internally, none of that is something thats become a proposal."

Earlier this year, Transportation Secretary Ray LaHood floated the idea of a vehicle miles tax charge as a replacement for the fuel tax, only to be rebuked by the White House.

Sen. Barbara Boxer, D-Calif., chairwoman of the Environment and Public Works Committee, said she could not support such a fee, even though Congress will be looking at "a new sustainable source of funding" as it develops a new highway bill.

"It seems to me, within the VMT realm, without getting into those things that I would not vote for - which is meters in your car and Big Brother checking in on you - I wouldnt have any of that," she said.

Boxer, who has three hybrid cars, acknowledges she said is an example of why the fuel tax alone cannot meet the nations transportation funding needs.

"I drive a hybrid . . . and I am not paying my fair share at the gas pump and Im using the roads," she said.

Sen. George Voinovich, R-Ohio, agrees that Congress has "to look at other methods . . . because we are moving toward reducing the number of vehicle miles traveled."

He said that for the near term nothing other than an increase in the fuel tax would generate "the dollars you need."

"Most people in the House and Senate are all worried about a vote on an increase in the gas tax," he said. "You cant do it without increasing the gas tax. Theres just no other alternative."

At a National Journal-hosted policy lunch, Deron Lovaas, federal transportation policy director at the Natural Resources Defense Council, said road pricing and vehicle mileage fees are slowly gaining traction because there is "a growing consensus about the need to price transportation goods which will help to reduce emissions and improve traffic flow."

Attorney Jack Schenendorf, who was vice chairman of one of two congressional commissions that studied transportation finance, said that a transition to a vehicle miles tax would take time.

"Theres no way to institute a large-scale pricing system in a two-year time frame," he said "It is going to take a lot longer than that."