As lawmakers scramble to reach agreement on a highway bill by week’s end, they appear to be leaving decisions on many big, politically tricky questions for last, such as whether to include language to approve the Keystone XL pipeline.

“We’ll take all that up after” the transportation-focused issues are resolved,  Environment and Public Works Chairman Barbara Boxer (D-Calif.) said off the Senate floor Wednesday.

On orders from their leadership, Boxer and House Transportation and Infrastructure Chairman John Mica (R-Fla.) are pushing to reach a deal on a transportation infrastructure bill by the end of the week.

Rep. Reid Ribble (R-Wis.), a member of the conference committee working on the bill, agreed that decisions on Keystone and other non-transportation-related issues would be left for last with leaders expected to weigh in.

“Those [transportation issues] are difficult enough,” Ribble said. “I think, quite frankly, on Keystone, that is probably going to be above everybody’s pay grade where leadership is going to get engaged.”

As transportation talks are ramping up, Congress also is working to pass legislation that would prevent an increase in student loan interest rates, which Democrats would like to do in conjunction with the transportation bill.

“It would be easier if you address them together,” a Senate Democratic aide said. “That would definitely be our preferred approach.”

However, no decisions have been made, and Republicans and Democrats remain at odds over how to pay for the both measures.

While the two issues will not necessarily be linked, both have the same deadline, with student loan interest rates set to double July 1, the same day transportation funding will cease to flow unless Congress either puts in place a new transportation measure or an extension of the current law.

Along with Keystone — a 1,700-mile pipeline from Canada to the Gulf Coast that the GOP has been pushing to include on the final bill — Republicans have also called for a provision that would prohibit the Environmental Protection Agency from setting enforceable safeguards for coal ash as well as inclusion of the Restore Act, which would send most of the fines from the 2010 BP oil spill to Gulf Coast states.

But before addressing those issues, Republicans want to reach agreement on reforms, including easing of environmental regulations in an effort to allow infrastructure projects to be completed more quickly.

“Reforms are critical,” said Rep. Steve Southerland (R-Fla.), also a member of the conference committee. “No one in America believes that it should take 14 years” to complete a highway project.

The GOP is also calling for the consolidation of transportation programs and a scaling back of transportation enhancements, which is the primary source of funding for bicycle and pedestrian projects.

Meanwhile, conference committee members are set for series of meetings in order to try to reach a deal on a bill by their self–imposed deadline of week’s end.

“Starting today we are meeting for hours, and we’ll let you know when we have an agreement,” Boxer said.

Boxer said she was meeting “with the principals” but declined to provide any other details.

The bicameral, bipartisan conference committee is made up of 47 lawmakers and charged with negotiating a transportation package that can pass both chambers. The panel will have to decide soon whether an agreement can be reached or whether it should instead pursue a short-term extension of current law.

Some lawmakers, such as Rep. Fred Upton (R-Mich.), were not optimistic that a deal could be reached.

“My suspicion is that we end up with a six-month extension,” Upton said.

The stakes are high, with billions of dollars for transportation projects and related jobs around the nation riding on Congress’ ability to pass a bill, said Senate Majority Whip Dick Durbin (D-Ill.), who is a member of the conference committee.

“If we do an extension, it kills the construction season in states that know what winter is all about,” said Durbin, who blamed Republicans for the stalemate.

In March, the Democratic-run Senate passed, 74-22, a surface transportation bill that would extend programs for two years and provide $109 billion for projects. After failing to pass a GOP House version, Republican leaders are still trying to negotiate a bill that can win support of freshman conservatives in their caucus. Tea-party-affiliated freshmen in particular have been skeptical of the need for a highway bill and the spending in the Senate package.

“Sixty-one thousand highway workers in Illinois will have their jobs in jeopardy if Speaker Boehner refuses to call up the Senate bill,” Durbin said.

Not only does an extension threaten jobs, Durbin argued, but it also cripples the nation’s ability to finance improvements and expansions of transportation infrastructure that is at capacity.

“What we are doing now is limping along with existing projects, with no promise of long-term funding, so any major project in my state is put on hold,” Durbin said. “If we had, at least, a two-year agreement, that project goes forward.”

Durbin demurred when asked whether he would oppose an extension, if that were all Congress could achieve.

“I am not saying I wouldn’t vote for an extension,” Durbin said. “I would do it pointing a finger to the House.”

Still, Durbin praised GOP Illinois Reps. Judy Biggert and Robert Dold, who have voiced their support for the bipartisan Senate passed transportation bill.

He sent a letter last week to the GOP Members of the Illinois delegation, calling on them to demand a House vote on the bipartisan Senate transportation bill.