Lawmakers are scrambling to come up with a Plan B to allow federal highway and transit spending to continue if the Senate bipartisan infrastructure agreement is not passed by the House before the law authorizing that spending expires Sept. 30.

House Democratic leaders say they still plan to bring the bipartisan infrastructure bill, which includes language reauthorizing federal highway and transit programs, to the floor on Sept. 27.

House Majority Leader Steny H. Hoyer said the vote could be Monday or Tuesday. But the bill’s fate is in question because of a standoff between Democratic moderates and progressives over that bill and the $3.5 trillion reconciliation measure.

Moderates are threatening to withhold support on the larger bill if they don’t get their Sept. 27 vote. Progressives are threatening to vote against the infrastructure plan until that larger measure passes both the House and Senate.

Speaker Nancy Pelosi, however, remained bullish late Wednesday. “We are on schedule,” she said. “And we’re calm and everybody’s good and our work’s almost done.”

Still, the potential that the bipartisan bill might fall short leaves the highway authorization, which is already a one-year extension of the 2015 law, in limbo.

Little GOP help

Republicans aren’t expected to provide much help. House Minority Whip Steve Scalise sent out an alert Wednesday urging Republicans to vote against the measure. Meanwhile, the chairman of the moderate Problem Solvers Caucus, Rep. Brian Fitzpatrick, R-Pa., predicts that a minimum of 10 Republicans will vote for the infrastructure bill, with others considering signing on based on what Democrats do.

House Transportation and Infrastructure Chairman Peter A. DeFazio, said Wednesday that his staff is drafting ideas, “but we don’t have a clear path” to reauthorization. “Anything we put through the House has to get past the 60-vote hurdle in the Senate,” the Oregon Democrat said.

DeFazio’s bill to reauthorize spending for surface transportation, as well as drinking water and wastewater projects, passed the House on July 1, but Senate negotiators crafting the bipartisan infrastructure bill largely ignored it, opting instead to fold language approved by the Senate committees of jurisdiction, including the Environment and Public Works Committee, which has jurisdiction over highways.

‘A plan afoot’

That committee’s chairman, Thomas R. Carper, D-Del., told reporters Wednesday that “there’s a plan afoot” to reauthorize the bill but gave little detail on what that plan is.

“The last thing we need is for Oct. 1 to roll around and the surface transportation projects all over the country — roads, highways, bridges — come to a halt, a screeching halt, because the federal government has not extended the authorization for such projects,” he said. “So it’s critically important that we address that.”

DeFazio said that without a reauthorization, the DOT will send out a notice early in October delaying reimbursements to states for projects that have already been completed.

“That would be very detrimental to the economy,” he said. “So we can’t let that happen.”

If the Senate bill fails in the House, that wouldn’t necessarily kill the bill, according to Jeff Davis of the Eno Center for Transportation, a think tank. The House could vote again on an identical motion, or a motion to concur with an amendment, the next day, “or anytime, as often as necessary until one passes.”

But if Congress doesn’t meet the Sept. 30 deadline, Davis wrote in an email, a few things will happen rapidly: There will be no new contract, or budget authority, to pay employees of Department of Transportation agencies such as the Federal Highway Administration, the Federal Motor Carrier Safety Administration and the National Highway Traffic Safety Administration, so those not deemed essential to the protection of life or property, or those not confirmed by the Senate, will be furloughed.

Davis said that means those who write checks to state transportation departments will be among those furloughed, which means those agencies will lose between $220 million and $250 million a day.

Susan Howard, program director for transportation and finance at the American Association of State Highway and Transportation Officials, said if the surface transportation law were to expire, it would temporarily halt state highway projects as well as the business that those projects flow to, such as contract work, construction and architecture.

“Everything would kind of freeze,” she said.