WARREN, Mich. (AP) — Republican presidential candidate John McCain acknowledged at a General Motors Corp. plant Friday that he disagrees with domestic automakers on whether states should have the ability to set more stringent tailpipe emissions — an issue the companies have vigorously opposed.

Responding to questions after an invitation-only, town-hall meeting at GM's Technical Center, the Arizona senator said "we have a disagreement" on whether states such as California should receive permission from the government to set tough greenhouse gas limits on cars, trucks and sport utility vehicles.

"It's hard for me to tell states that they can't impose ... whatever standards that would apply within their own states," McCain said during the meeting with about 500 employees and after a peek at GM's plug-in electric car, the Chevrolet Volt, with GM executives.

The Environmental Protection Agency in December blocked efforts by California and about 16 other states to impose tough regulations on emissions. California officials have said their program would cut emissions by one-third in new vehicles by 2016 and increase fuel efficiency to about 36.8 mpg.

EPA officials said the energy bill signed by President Bush last year would boost fuel economy standards to at least 35 mpg by 2020 while providing a national approach for reducing emissions. Automakers agree with the Bush administration and say a national approach would allow the industry to aim for one standard. They say the California rules would cost them billions of dollars and hurt their bottom line.

GM Chairman and CEO Rick Wagoner told reporters after the town-hall meeting that current fuel economy standards are challenging enough, and the automaker wants to focus on them. "It would be our preference to have a single national standard," Wagoner said.

The Senate Appropriations Committee is expected to consider a proposal Tuesday to provide low-interest loans to automakers to upgrade their plants to be able to build hybrids, plug-in hybrids and advanced vehicles. Democratic candidate Barack Obama has supported the provision while McCain has opposed it, arguing that his tax credits for plug-in hybrids and a $300 million battery prize would accomplish the same goal.

Both McCain and Obama have expressed support for California's efforts to implement their standards. Obama's campaign said it supported a package of incentives for the auto industry to help the companies develop more fuel-efficient vehicles to meet new standards.

Obama on Friday released a letter to the United Auto Workers outlining his policies for the industry, which include low-interest loans to help automakers modernize their plants to build advanced vehicles and a 10-year, $150 billion program for green manufacturing.

Obama's economic adviser, Jason Furman, said in a statement that Obama supports the right of states to set a higher standard but hopes that a strong national standard emissions standard makes that unnecessary. Furman said during a conference call with reporters that the senator's proposal of tax credits, loan guarantees and other incentives would help the industry meet tougher standards.

The auto industry and its allies in Congress have said allowing states to adopt their own requirements on tailpipe emissions would create inconsistent and competing fuel economy rules throughout the country that would undermine their industry.

"I don't think much of the idea of several sets of standards that the auto industry has to make and I think if you're trying to destroy the auto industry, that's a pretty good way of beginning," said Rep. John Dingell, a Dearborn Democrat who chairs the House Energy and Commerce Committee.

At the GM facility, McCain told the audience that he hopes auto executives and state lawmakers will be able to work out an acceptable solution, and the Volt would eventually "make this discussion academic."

He offered glowing praise for the rapid development of the car and its potential to help reduce U.S. dependence on foreign oil as well as fossil fuels.

"The key, integral, vital part of our ability to eliminate our dependence on foreign oil will be directly related to that sign over there," McCain said, pointing to a Volt sign.

During his tour with executives, he examined and got into a model in development, which GM plans to have on the market by 2010.

McCain has called for incentives for next-generation vehicles and the need to move to new technology such as the Volt.

The GM officials also showed McCain a cutaway mock-up of the Volt's power system and innovative battery that would let it drive about 40 miles without using any gasoline.

McCain's GM visit comes the same week the company announced a combination of cuts, borrowing and asset sales that would raise $15 billion to weather the recent slump in U.S. auto sales and the rapid shift from trucks to cars.

Wagoner said he hopes GM's recent turnaround efforts show McCain and Obama that the company is capable and committed to fixing itself — and all it seeks is their support and understanding.

"We're not looking for people to bail us out," he said.

Kathy Doher, a 36-year employee of GM, said she attended the event because of her interest in such issues as the economy and the state of the auto industry. While undecided, she also was interested in attending a meeting with a presidential candidate.

With retirement in sight, it's not about her personal job security, but she said, "I'm obviously concerned about just turning the company around in North America."

"We really do have some great products out there that can compete with the best," said Doher, an assistant to the vice president of North American engineering.

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Associated Press writers Ken Thomas in Washington and David N. Goodman in Detroit contributed to this report.