WASHINGTON -- Automobile tire pressure has become an unlikely issue in the U.S. presidential campaign, as John McCain mocked Barack Obama's recommendation that drivers keep their tires well-inflated -- and then Obama taunted his Republican rival for admitting that the practice saves gasoline.

Both candidates have been vying to persuade voters that they have the best plans to bring down high gasoline prices, which top the list of American voters' concerns. Most of the debate has centered on nuclear energy, offshore drilling and tapping strategic petroleum reserves. But on Wednesday, the talk was about tires.

Discussing the air-pressure issue during an appearance Tuesday night, McCain said he was not opposed to Obama's suggestion. "And could I mention that Senator Obama a couple of days ago said that we ought to all inflate our tires, and I don't disagree with that. The American Automobile Association strongly recommends it, but I also don't think that that's a way to become energy independent."

Obama had noted that keeping tires should be part of any comprehensive plan to reduce reliance on imported oil.

In mocking Obama, McCain said at a motorcycle rally in Sturgis, South Dakota: "My opponent doesn't want to drill, he doesn't want nuclear power, he wants you to inflate your tires."

Obama seized on both statements Wednesday in trying to portray McCain as a flip-flopper. It "will be interesting to watch this debate between John McCain and John McCain," Obama said as he campaigned in Indiana with Sen. Evan Bayh, widely considered a top-tier candidate for running mate.

The energy sparring continued as McCain criticized Obama for not fully embracing nuclear power as part of a comprehensive energy solution. "He's out of touch," McCain said while campaigning in Ohio on Wednesday.

McCain called on Congress to return from its summer recess to address immediately the U.S. energy crisis, though he missed numerous energy-related votes in the Senate last year.

As president, he would tell members of Congress "not to leave town, not to take their vacation or pay raise" until they passed legislation to ease the burden of high energy prices on consumers, he said.

The idea of forcing Congress to deal with energy issues could open the Arizona senator to charges of hypocrisy. The liberal-leaning League of Conservation Voters gave McCain a "zero" rating for 2007, saying he had missed all 11 votes related to such critical energy topics as automobile fuel economy, offshore Virginia drilling, refinery construction, renewable electricity mandates, energy efficiency, liquefied coal, support for biofuels.

Last weekend, the House and the Senate adjourned amid calls from House Republicans for a vote on an energy bill that would expand more domestic oil drilling. In the Senate, a debate was pending on a proposal by a bipartisan group of 10 to boost taxes on oil companies while allowing a limited expansion of oil drilling off the coast of Southern states.

Both McCain and Obama have signaled an openness to such a compromise, but they say they need to review details of the proposal. Obama had earlier steadfastly opposed offshore drilling, citing general agreement among energy experts that the new supplies would be years in reaching gasoline pumps.

McCain has to address another tricky economic issue Thursday, taking up the issue of possible job losses due to the closure of a German-owned DHL shipping site in Ohio, the result of a corporate merger aided by his campaign manager during his work as a lobbyist.

In 2003, McCain campaign manager Rick Davis lobbied Congress to accept a proposal by DHL to buy Airborne Express, which kept its domestic hub in Wilmington in southwest Ohio.

In announcing a restructuring plan in May, DHL said it planned to hire United Parcel Service to move some of its air packages, sending them through an airport in Louisville, Kentucky, and putting the Wilmington Air Park out of business. Some 8,000 jobs could be at stake, Wilmington officials estimate.

Davis took a leave of absence from his lobbying practice to work for McCain, a self-styled reformer who asked his campaign staff to disclose all previous lobbying ties and make certain they were no longer registered as lobbyists or foreign agents.

McCain on Thursday was to discuss DHL's plans with local officials and others affected by the potential job losses.

Meanwhile, McCain's campaign released a new TV ad questions whether Obama is ready to help American families. It goes on to criticize Obama as promoting higher taxes and more government spending, both leading to fewer jobs.

Obama rolled out a new ad that bids to reinforce the Democrat's theme that McCain represents a continuation of the unpopular policies of President George W. Bush.

As both candidates move toward their party conventions, which will mark the home stretch toward the Nov. 4 presidential balloting, the campaign has become increasingly negative. The race remains relatively close, with the most recent Associated Press-Ipsos poll showing Obama with a six-percentage point advantage over McCain at 47-41.

Obama also led in two polls released Wednesday. A Time magazine poll showed Obama ahead of McCain 46 percent to 41 percent. A CBS News poll put Obama at 45 percent to 39 percent for McCain.

The Obama ad was the latest in a series of tit-for-tat exchanges on American television screens but notable this time for not contrasting criticism of McCain with praise for Obama.

Obama's ad attacks McCain's support of Bush tax breaks for wealthy Americans, "giveaways" to the oil industry and tax incentives to companies that move operations overseas.

McCain's new response asks, "Is the biggest celebrity in the world ready to help your family?," attempting to capitalize on a sense among U.S. voters that Obama is overexposed.

With the Nov. 4 election still three months away, 48 percent of Americans say they are hearing too much about Obama, according to a poll released Wednesday by the nonpartisan Pew Research Center. Just 26 percent said the same about McCain.

According to an ongoing Pew study, Obama has appeared in more news stories this year and more people say they have heard more about him than McCain.