During the current push for multi-year transportation bill, the Senate has been known more for its spirit of bipartisanship than any visionary policy advances. Now that the bill has hit the Senate floor, however, it’s getting a little ugly.

Republican Senators have proposed a number of contentious amendments in order to stall the bill’s passage. Senator Barbara Boxer, whose Environment and Public Works Committee got the ball rolling on the bill last December, says these amendments are “ridiculously unrelated.” For example, Rand Paul has proposed an amendment that would cut off American aid to Egypt until the 19 American civilians there are allowed to leave. Other amendments try to thwart the president’s compromise on insurance coverage for contraception.

In response, majority leader Harry Reid has “filled the tree,” proposing enough amendments to fill all available slots before any other senator can do the same. The move triggers another cloture vote, this time on specific amendments themselves, rather than the entire bill. The vote, scheduled for tomorrow, will decide whether the Senate goes forward in combining the EPW, Commerce, Banking, and Finance titles into a single bill.

Boxer, who had been generally upbeat about the Senate bill’s prospects for passage, is getting frustrated with the delay tactics, according to The Hill:

“Right now, there is no path forward,” Boxer said even as she vowed to continue pushing for a clean vote on the transportation proposal.

“I don’t see it,” Boxer said. “It’s one of those things where people just say ‘I don’t care. We’re not going to (approve) this bill.'”

The delay also means that other noteworthy amendments will have to wait until the current quarrel dies down. Two Democratic Senators and two Republicans have just cosponsored an amendment that would restore dedicated funding to the Recreational Trails program, protecting biking and walking programs that the underlying bill undermines. The Cardin-Cochran amendment, which gives metropolitan areas more control over federal funds, is also picking up cosponsors from both parties.

The fate of these two amendments and more — T4America has a handy page to track their progress — hinge on Senate leadership setting aside parliamentary scheming, and soon.

Reid’s last cloture vote on the transportation bill succeeded by a healthy 85-11 margin, but it remains to be seen whether that margin will shrink in future votes as a result of this maneuvering. Filling the tree is tactic that Reid has developed quite a reputation for using, and it has sometimes come at a political cost. Reid filled the tree to ensure passage of President Obama’s health care bill, and to protect the EPA’s authority to regulate greenhouse gases. Reid also filled the tree while attempting to protect the provision that ended the military’s “don’t ask-don’t tell” policy — and it cost him the support of moderate Republican Susan Collins, a crucial swing vote, in the end.

Update: In her remarks on the Senate floor this morning, Sen. Boxer did say that she now sees a path forward, though it is still obscured by some “bizarre, extraneous amendments.”