House passage of another 90-day extension of the highway program sets the stage for another nail-biter.

Including today, there are 31 working days left before the current extension expires. The extension the House passed on Wednesday would give the House and Senate until the end of the fiscal year in September to negotiate the details of a highway bill that lasts longer than 90 days.

The degree of difficulty for achieving this is quite high.

Transportation Secretary Ray LaHood, speaking at a conference hosted by Politico, said that although he wants a highway bill, he knows it won't pass.

First, both chambers must appoint conferees.

"The fact that the House voted to take a step forward on a surface transportation bill is encouraging, as long as they follow through and immediately appoint conferees," said Sen. Barbara Boxer, D-Calif., in a statement.

Boxer chairs the Senate Environment and Public Works Committee. She and the ranking Republican on the committee, Sen. James Inhofe of Oklahoma, were the main authors of the two-year, $109 billion highway bill the Senate passed in a bipartisan vote last month.

Boxer said Senate Majority Leader Harry Reid, D-Nev., has agreed to appoint conferees as soon as he can.

"The final bill must be truly bipartisan so it can pass both Houses of Congress," Boxer said.

Bipartisanship will be difficult to get.

The House extension contains a provision authorizing construction of the Keystone XL pipeline, which the Obama administration opposes until the project gets State Department clearance.

Also difficult is the all-important question of funding.

Both chambers want to keep highway funding at current levels, which requires more money than the Highway Trust Fund will produce. But neither chamber is willing to raise fuel taxes to cover the shortfall, and each has a different approach to how the difference should be made up.

The House has proposed collecting fees for new oil and gas drilling, and the Senate has proposed a bundle of funding transfers and offsets.

Rep. John Mica, R-Fla., is chairman of the House Transportation and Infrastructure Committee and main author of the five-year, $260 billion highway bill that the House has not been able to pass.

He said he intends to push for as many provisions of that bill as he can get into a House-Senate conference measure.

In fact, both Mica's bill and the two-year Senate bill contain reforms that would make big changes in the way the Department of Transportation does business. They both consolidate DOT programs, expedite project delivery, eliminate earmarks and give states more say in how to spend federal dollars.

If the conferees can overcome their differences over funding and the term of a highway bill, they just might be able to include substantive, long-sought reforms of the federal transportation program.

The alternative would be to extend the program until after the November election.

That would put it in line behind showdown issues such as the expiration of the Bush tax cuts at the end of the year, and the beginning of almost $1 trillion in spending cuts under the Budget Control Act of 2011.