WASHINGTON — A House-passed bill to speed $14 billion in loans to Detroit's automakers stands on shaky ground in a bailout-weary Congress, undermined by Republican opposition that could derail the emergency aid in the Senate.

Republicans are challenging lame-duck President George W. Bush on the proposal, arguing that any support for the domestic auto industry should carry significant concessions from autoworkers and creditors and reject tougher environmental rules imposed by House Democrats.

The House approved the plan late Wednesday on a vote of 237-170. It would infuse money within days into cash-starved General Motors Corp. and Chrysler LLC. Ford Motor Co., which has said it has enough cash to make it through 2009, would also be eligible for federal aid.

Butler County's two congressmen, Republican Phil English and Democrat Jason Altmire, voted for the measure.

Altmire issued this statement about his vote:

"I voted for this legislation because it met the three conditions that were absolutely critical to my support: it ensures assistance to American automakers is structured as a loan, not a giveaway; it uses funds that have already been designated to help the auto industry and therefore does not spend any new taxpayer money; and it requires the automakers receiving assistance to fundamentally restructure themselves and change their business models so they can successfully compete in the global economy.

"The collapse of even one of America's Big Three automakers could cost thousands of Pennsylvanians and millions of Americans their jobs.

"Doing nothing and seeing one or all of the Big Three automakers fail would be far costlier to taxpayers than providing the industry with a $14 billion loan which they will have to repay with interest."

Supporters cited dire warnings from GM and Chrysler executives, who have said they could run out of cash within weeks, and concerns that a carmaker collapse would erase tens of thousands of jobs and jolt an already bleak economy.

Democrats and the Bush White House hoped the Senate would vote on the legislation as early as Thursday. But based on concerns raised by GOP senators — and a still-uncertain level of support even among Democrats — they had a lot of work to do.

A leading Senate Republican opponent said Thursday that he cannot back spending $14 billion of taxpayer money on a plan that would call for a restructuring of the industry, but which fails to detail just how that would be accomplished.

"I think that is putting the cart before the horse and isn't reponsible in terms of tax dollars," Sen. David Vitter of Louisiana said on CBS's "The Early Show."