The legislation, approved 237-170 by the House, would provide money within days to cash-starved General Motors Corp. and Chrysler LLC. Ford Motor Co., which has said it has enough to stay afloat, would also be eligible for federal aid.
Republicans were preparing a strong fight against the aid plan in the Senate, not only taking on the Democrats but standing in open revolt against their party's lame-duck president on the measure.
The Republicans want to force the companies into bankruptcy or mandate hefty concessions from autoworkers and creditors as a condition of any federal aid. They also oppose an environmental mandate that House Democrats insisted on including in the measure.
House Speaker Nancy Pelosi said the House-passed bill represented "tough love" for U.S. auto companies and "giving a chance - this one more chance - to this great industry."
The White House, struggling to sell the package to congressional Republicans, said earlier that a carmaker bankruptcy could be fatal to the auto industry and have a devastating impact on workers, families and the economy.
"We believe the legislation developed in recent days is an effective and responsible approach to deal with troubled automakers and ensure the necessary restructuring occurs," said Dana Perino, the White House press secretary.
But the measure faces a difficult road in the Senate, where it needs 60 votes to advance. Rank-and-file Senate Republicans skewered the bill during a closed-door luncheon with White House Chief of Staff Josh Bolten, who was dispatched to Capitol Hill to make a case for the rescue package.
Besides providing cash for the auto companies, it would create a government "car czar," to be named by President George W. Bush to dole out the loans, with the power to take back the money and force the carmakers into bankruptcy next spring if they didn't cut quick deals with labor unions, creditors and others to restructure their businesses and become viable.
"To give up on the auto industry now would be to condemn the American economy at one of its most vulnerable periods in our economic history to a degree of further hurt," said Rep. Barney Frank, D-Mass, the Financial Services Committee chairman.
Behind the scenes, Senate Democratic and Republican leaders scrambled for a deal that would allow votes on the bill on Thursday. Some GOP senators were demanding votes on an alternative that would order the automakers to take specific actions to restructure - including steep wage cuts and debt restructuring - in return for any federal money.
Opposition from Republicans reflected the tricky task of pushing yet another federal rescue through a bailout-weary Congress, with Bush's influence on the wane.
"People realize that this bill is an incredibly weak bill (and) is the product of an administration that wants to kick the can down the road and let somebody else deal with it," said Sen. Bob Corker, R-Tenn.
The auto aid debate was replete with echoes of the tense, early-October drama surrounding the $700 billion Wall Street bailout, when a marathon set of negotiations yielded a much-celebrated deal that came apart quickly amid GOP opposition and public outrage. That bill ultimately passed after much arguing, cajoling, threatening and lobbying among lawmakers, and Bush signed it.
Before passing the auto measure Wednesday, the House voted to add language requiring that banks that are bailed out by the government report quarterly on how much they have increased or decreased lending.
In the Senate, opposition to the auto rescue wasn't limited to Republicans.
Democratic Sen. Max Baucus of Montana announced he was against the measure because of a provision to bail out transit agencies that were involved in transactions that are now considered unlawful tax shelters.
House Republicans swiftly voiced their opposition and called for a plan that would instead provide government insurance to subsidize new private investment in the Big Three automakers, demand major labor givebacks and debt restructuring at the companies and encourage them to declare bankruptcy.
Under the House-passed measure, the carmakers would have to submit blueprints on March 31 to the industry overseer showing how they would restructure to ensure their survival, although they could be given until the end of May to negotiate with the government on a final agreement.
The carmakers initially asked Congress for $25 billion, then returned two weeks later to plead for as much as $34 billion. But with the White House refusing to dole out new spending for the Big Three, congressional Democrats agreed to use an existing program that was to help carmakers retool their factories to make more fuel-efficient cars.
That fund yielded only $15 billion in emergency loans, and when negotiators agreed to leave some money in the environmental program, the amount fell to $14 billion.
Democrats agreed to scrap language - which the White House had called a deal-breaker - that would have forced the carmakers to drop lawsuits challenging tough emissions limits in California and other states. But they kept a provision to force the automakers to abide by those states' limits - a kind of consolation prize for environmentalists, who already were livid at the raid of the fuel-efficiency program.
Senate Democrats unveiled a nearly identical measure that omitted the requirement, but that bill still faced long odds. At the White House, Deputy Chief of Staff Joel Kaplan said the Bush administration would work with President-elect Barack Obama's team on choosing industry czar.
Obama defended the auto bailout as necessary given the threat a potential Big Three collapse could pose to an already battered economy.
"As messy as it may be, I think there's a sense of, 'Let's stabilize the patient,"' he said in an interview published in Wednesday's Chicago Tribune and Los Angeles Times.
The car czar would have say-so over any major business decisions by the automakers while they were taking advantage of federal aid, with veto power over any transaction of $100 million or more. The companies - including the private equity firm Cerberus, which owns a majority stake in Chrysler - would have to open their books to the government overseer.
And if Chrysler defaulted on its loan, Cerberus would be responsible for reimbursing the government.
The measure also would attach an array of conditions to the bailout money, including some of the same restrictions imposed on banks as part of the $700 billion Wall Street rescue. Among them are limits on executive compensation, a prohibition on paying dividends and requirements that the government share in future profits and taxpayers be repaid before any other shareholders.
Also included in the bill is an unrelated pay raise for federal judges.
Associated Press writers Ben Feller, Andrew Taylor and Jim Abrams contributed to this report.