WASHINGTON — The top executives of Detroit’s auto industry today asked House Speaker Nancy Pelosi to fund a $25 billion loan program to help the companies modernize their plants, arguing the sluggish American economy could affect thousands of workers.

The leaders of General Motors Corp., Ford Motor Co., and Chrysler LLC met with Ms. Pelosi and said in a letter released today that they are committed to building more fuel-efficient vehicles, but the industry faces significant challenges because of the soft American economy. They called it “the most difficult business environment for our domestic auto industry in over thirty years.”

Several factors, including the housing crisis, the credit crunch, rising commodity prices and the rapid change in gas prices have affected sales, the executives wrote.

“The potential adverse effects of the above combination of factors — factors outside our direct control — have the potential to severely impact tens of thousands of employees and have a lasting effect on industrial production in the U.S.,” they wrote in the letter dated yesterday.

GM’s chairman and CEO, Rick Wagoner, Ford’s CEO, Alan Mulally, and Chrysler’s chairman and CEO, Robert Nardelli, who signed the letter, met with Ms. Pelosi for about 45 minutes and spoke with other congressional leaders separately.

Mr. Nardelli said the conversations were “very encouraging, very candid, very straightforward” and Mr. Mulally told reporters they were making “tremendous progress.” When Mr. Mulally was asked if the loans represented a bailout, he said, “I would characterize it as an enabler.”

Earlier in the day, Senator Harry Reid, Democrat of Nevada, said it was “extremely important” that Congress provide funding for the loans. The proposal is expected to be considered before Congress adjourns later this month.

In the letter, the executives asked for full funding for the $25 billion loan program and an expansion of the program “to allow for more investments in fuel efficient vehicles and components to qualify for direct loans.”

In a separate letter, 10 governors, led by Michigan Governor Jennifer Granholm, urged congressional leaders to fully fund the loan program.

“Doing so will help create green jobs, reduce greenhouse gas emissions, and move our country toward energy independence,” the governors wrote. Nine of the governors are Democrats and one — South Dakota Governor Mike Rounds — is a Republican.

Missouri Governor Matt Blunt, a Republican, also wrote a letter to his state’s congressional delegation highlighting benefits of the loan program.

Congress authorized $25 billion in loans in last year’s energy bill but has not funded the plan. Lawmakers devised the loans to help auto companies and their suppliers meet new fuel-efficiency standards of at least 35 miles per gallon by 2020, a 40% increase.

Since the energy law was signed, the Detroit companies have struggled with a slow economy, limited access to credit, and sluggish sales.

The government interest rate for the loans would be around 5%, providing about $100 million a year in savings for every $1 billion the companies receive in loans. The companies have poor bond ratings, meaning they would only qualify for double-digit interest rates on the open market.

“If we don’t keep the auto industry alive and provide them access to capital to make the changes that not only they want but the consumers want, they’re done. We’ve got to have that,” Rep. Fred Upton, Republican of Michigan, said at a conference on sustainable resources sponsored by Arizona State University.

Ford’s executive chairman, Bill Ford, who also participated in the Arizona State conference, said the industry was on “the cusp of a really interesting time” that will lead to alternative vehicles, but they will need help with the infrastructure to make it work.

“It will take a coordinated effort … really a partnership between government industry and the fuel providers to pick a path,” Mr. Ford said.