WASHINGTON —Detroit’s Big Three automakers pleaded with Congress Tuesday for a $25 billion lifeline to save their once-proud companies from collapse, warning of broader peril for the national economy as well.

It was an uphill battle, with the plan stalled on Capitol Hill amid opposition from Republicans and the Bush administration. But congressional leaders worked behind the scenes in an effort to hammer out a compromise that could speed some aid to the automakers before year’s end.

The executives of Chrysler LLC, Ford Motor Co. and General Motors Corp., as well as the United Auto Workers union chief, were pleading their case Tuesday afternoon before the Senate Banking Committee. A House panel was to hear from them Wednesday.

Majority Leader Steny Hoyer said Congress might have to return in December — rather than adjourning for the year this week, as expected — to push through an auto bailout.

“Dealing with the automobile crisis is a pressing need. We are talking about a lot of people … and a great consequence to our economy,” said Hoyer, D-Md. “Obviously we are going to be back here, we think, in December.”

The financial situation for the automakers grows more precarious by the day. Cash-strapped GM said it will delay reimbursing its dealers for rebates and other sales incentives and could run out of cash by year’s end without government aid.

In the Senate, leaders were focusing on a plan favored by the White House and GOP lawmakers to let the auto industry use a $25 billion loan program created by Congress in September — designed to help the companies develop more fuel-efficient vehicles — to tide them over financially until President-elect Barack Obama takes office.

However, House Speaker Nancy Pelosi, D-Calif., and other senior Democrats, who count environmental groups among their strongest supporters, have vehemently opposed that approach because it would divert federal money that was supposed to go toward the development of vehicles that use less gasoline.

Instead, they want to draw a separate $25 billion for the industry from the $700 billion Wall Street bailout — bringing the government’s total aid to the car companies to $50 billion.

A Senate vote on that plan, which would also extend jobless benefits, could come as early as Thursday, but aides in both parties and lobbyists tracking the effort privately acknowledge it doesn’t have the support to advance. Treasury Secretary Henry Paulson renewed the administration’s opposition on Tuesday.

Even the car companies’ strongest supporters conceded Tuesday that changing the terms of the fuel-efficiency loan program might be the only way to secure funding for them with Congress set to depart for the year and the firms in tough financial shape.

“While I believe we have to have retooling going into next year, if in the short run the only way we have to be able to get some immediate help is to take a portion of that, I would very reluctantly do that — but only because I believe President-elect Obama is going to be focused on retooling and on a manufacturing strategy next year,” said Sen. Debbie Stabenow, D-Mich.

The White House said the government shouldn’t send any more money to the struggling auto industry on top of the already-approved loans.

“We don’t think that taxpayers should be asked to throw money at a company that can’t prove that it has a long-term path for success,” said White House Press Secretary Dana Perino.

Sen. Mitch McConnell, R-Ky., the minority leader, said that redirecting the existing loans was “a sound way to go forward,” and that he was working with Democratic Leader Harry Reid of Nevada to set a vote on such a plan.

“The auto industry obviously is very important, very important to my state, but there is a way to do this,” said McConnell, who has two Ford plants and a GM plant in his state.

Paulson, testifying on the House side, defended the administration’s handling of the massive $700 billion bailout for the financial industry and said it should remain off-limits for Detroit, no matter how badly the automakers need help.

“There are other ways” to help them, he said.

At the same time, he testified, “I think it would be not a good thing, it would be something to be avoided, having one of the auto companies fail, particularly during this period of time.”

The industry mounted a feverish lobbying effort to secure funds they said were vital to their survival — and the health of the broader economy.

In an e-mail marked “urgent” and sent to owners of GM vehicles, Troy A. Clarke, president of GM North America, pleaded with them to e-mail their representatives in the House and Senate in support of a “bridge loan” for the industry — and ask their friends and family to do the same.

“Despite what you may be hearing, we are not asking Congress for a bailout but rather a loan that will be repaid,” Clarke said in the message.

That argument could be vital as bailout fatigue threatens to sap support for the carmaker aid.