This post was most recently updated at 12 p.m.

This month, a congressional impasse shut down parts of the nation’s airline transportation system. Next month, according to President Obama, it could be highways and other mass transit projects.

Hoping to head off a work stoppage, President Obama on Wednesday called on Congress to approve a funding extension to the Surface Transportation Bill, which is due to expire at the end of September. If the bill is not extended, Obama said, 4,000 workers would be immediately furloughed without pay and 1 million workers could be out of a job over the next year.

“All of them would be out of a job just because of politics in Washington,” Obama said during an appearance in the Rose Garden on Wednesday. “It’s just not acceptable. It’s inexcusable to put more jobs at risk in an industry that is one the hardest hit over last decade. It’s inexcusable to cut off new investments at a time when our highways are choked with congestion.”

Congress is divided on how to extend the highway funding. The Republican controlled House is considering a six-year, $230 billion bill that would be paid for with fuel taxes. The Democratically controlled Senate proposal would last only two years, and cost $109 billion.

A spokesman for House Majority Leader John Boehner (R-Ohio) accused Obama of “scare tactics” in speculating that lawmakers would allow the funding to run out, and said the White House has not offered its own proposal for how to pay for critical infrastructure without adding to the deficit.

“Aside from the President today, no one has suggested the highway bill will be allowed to expire,” the spokesman, Brendan Buck, said in a statement. “These types of scare tactics are irresponsible, transparently political, and needlessly add uncertainty to our economy.

“...Republicans have proposed a long-term reauthorization that reduces bureaucracy, focuses on legitimate federal transportation priorities, and meets a basic principle of spending only what we take in (from the gas tax).”

Obama tied the transportation funding to his renewed effort to boost job creation. He is scheduled to speak to labor groups in Detroit on Monday and to deliver a major jobs speech later next week.

Obama also demanded that Congress fund a clean, long-term extension of the Federal Aviation Administration reauthorization bill, which is set to expire in mid-September. Congress approved a temporary funding measure after a two-week shutdown left 74,000 workers unemployed in late July and early August.

As he has done in several speeches this month, Obama sought to cast Congress as standing in the way of his effort to create jobs and boost the economy.

“If this story sounds familiar, it’s because we heard it before,” Obama said. “Just a few weeks ago, Congress refused to act on another bill, typically a routine bill.”

In a show of unity, Obama was flanked in the Rose Garden by leaders of groups that haven’t always seen eye-to-eye: Transportation Secretary Ray LaHood, AFL-CIO chief Richard Trumka and U.S. Chamber of Commerce Chief Operating Officer David Chavern, as well as several workers who would be affected if funding disappeared.

The funding measures have been routinely extended in the past. The highway funding bill has been extended seven times in the last two years, Obama said. Administration officials chided Congress for playing politics.

“Normally, we wouldn’t have to worry about this, but the FAA debacle shows we can’t assume people won’t use it as a chance to drive their agenda,” White House Communications Director Dan Pfeiffer said on his Twitter account Wednesday morning.