WASHINGTON (AP) — Lawmakers are under pressure to find a way to keep federal aid flowing to highway and transit programs beyond the end of this month after a transportation bill failed to clear a procedural hurdle in the Senate.

The government's power to spend federal Highway Trust Fund money on transportation programs and to levy federal gas and diesel taxes that support the trust fund are due to expire March 31. If that were to happen, states could have difficulty paying for construction projects already in progress and would likely be reluctant to commit to new projects, lawmakers and transportation interest groups said.

As many as 800,000 workers could lose their jobs, said Senate Majority Leader Harry Reid, D-Nev., trying unsuccessfully to persuade Republicans senators to limit debate on the $109 billion bill, which would overhaul transportation programs and keep them from running out of money for about two years.

Leaders in both the House and Senate are struggling to pass bills that would lay out blueprints for federal transportation programs for the next several years. The Senate bill has bipartisan support, although it has stalled in recent weeks as Republicans have pressed for offer amendments on unrelated issues such as the Keystone oil pipeline and pollution controls for industrial boilers.

Senate Democrats fell eight votes short of a 60-vote threshold required to limit debate and move forward with the bill. All but two Republicans — Sen. Olympia Snowe of Maine and Scott Brown of Massachusetts — voted against a motion to limit debate.

Reid and other accused Republicans of political obstruction. GOP senators said they were trying to preserve their right to offer amendments. Moments before the vote, Senate Minority Leader Mitch McConnell, R-Ky., offered Reid a compromise as the two lawmakers stood on the House floor. The proposal would cap amendments to a list of about 30 that have been offered by Democrats and Republicans.

"This is a bill that is not going to be stopped. It has broad bipartisan support," McConnell said. "We anticipate being able to wrap it up."

But Reid, who said he would look at the proposal, complained: "I don't know why everything we have to do has to be a fight." He noted that action on bill had previously been stalled by a GOP amendment to provide religious exemptions to President Barack Obama's mandate that health insurers cover the cost of contraceptives.

The two leaders are continuing to negotiate on a list of acceptable amendments, and McConnell said he was optimistic an agreement can be reached.

In the House, Speaker John Boehner, R-Ohio, signaled he might be willing to take up the Senate bill in there, a possibility that could boost prospects for congressional passage before the March 31 deadline.

Asked at a news conference if the House might take up the Senate bill, Boehner said, "that is an option."

Boehner made a transportation bill the election-year centerpiece of the GOP's jobs agenda last fall when he unveiled its broad outlines. But support for the five-year, $260 billion House bill fell apart because conservatives thought it would spend too much money, and Democrats and some Republicans balked at policy changes they said would undermine mass-transit, weaken environmental protections and penalize union workers.

The last long term transportation plan passed by Congress expired in 2009. Lawmakers have kept programs going through a series of eight short-term extensions. The biggest difficulty has been that federal fuel taxes no longer raise enough money to pay for highway and transit programs at their current spending levels. As a result, Congress has had to repeatedly transfer money from the general treasury to the trust fund to keep it from running out of money. Despite previous cash infusions, the Congressional Budget Office estimates the fund will go broke sometime this winter unless Congress agrees to cut transportation spending or comes up with a way to pay for the programs.