A host of interest groups are already fighting it out for a slice of the next highway bill — expected to top out at nearly half-a-trillion dollars — but stakeholders across the transportation spectrum are united on one point: They want Congress to pass a mammoth reauthorization soon after it expires this fall, not wait another year and a half.

Smart-growth advocates, state officials, labor interests and major business groups have all signaled growing frustration in recent weeks with legislation moving through the Senate — at the White House’s behest — that would postpone the transportation reauthorization bill for 18 months beyond its Sept. 30 expiration date.

“We are fully aware that there probably will have to be some sort of extension, but 18 months is too long,— said Janet Kavinoky, the director of transportation infrastructure for the U.S. Chamber of Commerce and executive director for its Americans for Transportation Mobility Coalition.

If past highway bills are a guide, extensions will only beget more extensions, she said. “Eighteen months is basically saying we’re not going to do this bill for another couple of years.—

The growing chorus against an extension strengthens the hand of a determined House Transportation and Infrastructure Chairman James Oberstar (D-Minn.), who for months has been pushing the White House and the Senate to take up his six-year, $500 billion reauthorization bill.

The end result, according to one transportation lobbyist, will most likely be a late September negotiation that will result in a much shorter extension of the current law — such as six months.

That allows the administration the time it says it needs for the bill but also sets a new deadline closer to the line drawn in the sand by Oberstar, who at this point continues to oppose any extension beyond Sept. 30.

“It’s going to be somewhere between what Oberstar wants and what the Senate wants,— said the lobbyist.

The Obama administration insists it also wants to do a long-term bill — but only after it finishes health care and global warming. “We think the effort is too important, and too complex, to be rushed through Congress with so many competing priorities,— Transportation Secretary Ray LaHood said in a speech Monday.

At the heart of the delay is uncertainty over how to pay for the bill.

LaHood has made clear the administration won’t support a temporary hike in the gasoline tax — an idea pushed by a Congressionally mandated commission — during a recession, nor does it want to broker a compromise among the diverse interests that make up the transportation universe while it simultaneously tries to thread the health care needle.

But Kavinoky said an ambitious agenda is no excuse to punt on what should be a routine authorization. “There’s never going to be a convenient time to have the revenue debate,— she said. “We’ve known since 2005 that this deadline is coming.—

The unfolding political dynamic — which pits a powerful House chairman against several Senators, including Environment and Public Works Chairman Barbara Boxer (D-Calif.), acting on the White House’s behalf — has outside interest groups treading carefully, anxious not to aggravate either side.

“Getting in the middle of this sort of internecine thing between the House and Senate is nobody’s idea of a good time,— said David Goldberg, a spokesman for Transportation for America, a broad coalition formed last year to press smart-growth principles and sustainability in the reauthorization debate.

Nonetheless, powerful lobbies are making it increasingly clear they don’t want to wait for the debate. The chamber’s Americans for Transportation Mobility Coalition, which includes the American Road & Transportation Builders Association, the Associated General Contractors, and the American Public Transportation Association, earlier this month wrote every Member of Congress to urge that any extension be limited until the spring.

The Transportation Trades Department of the AFL-CIO, which represents 32 transport-related unions, also came out strongly against the 18-month plan in an appearance before the House Ways and Means Committee last week. Other groups acknowledge they are stepping up their outreach to lawmakers and staff — especially in the Senate — to push for faster action on reauthorization.

While outside interests are largely united against the administration’s 18-month plan, there is less unanimity on whether a short-term extension should be “clean— — as advocated by the Chamber of Commerce and the Senate — or include some reforms as a “down payment— on the longer-term bill.

Deron Lovaas, the federal transportation policy director for the Natural Resources Defense Council, argues that Congress should include some reforms with any extension, especially if, as expected, general revenue funds are to be transferred into the Highway Trust Fund, which operates under formulas tied to states’ contributions.

“You’re taking general fund taxpayer money now and spending it through a transportation program that’s widely recognized as broken and that’s wrong,— he said.

But another lobbyist said attempting to add a few targeted reforms to an extension bill is a recipe for disaster. “If you open it up to one provision, you’ve got to open it up to everyone, and then legislatively it gets completely bogged down,— the official said.