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Stepping up its response to attacks from Hillary Rodham Clinton, Barack Obama's campaign released a radio ad defending his independence from lobbyists and commitment to alternative energy.

The new spot (subscription), which began airing Thursday afternoon, accuses Clinton of playing "political games" while Pennsylvanians face real problems. "An economy in shambles, families struggling, gas prices close to four dollars a gallon," the ad says. "What's Hillary Clinton's answer? Misleading negative ads."

The ad's assertion that Obama is "the only candidate who doesn’t take a dime from oil company PACs or lobbyists" represents a slight distinction from his original TV ad, which came under fire from fact-checkers as well as the Clinton campaign. The ad goes on the offense as well, charging that "the Federal Election Commission reports that Clinton’s taken more from Big Oil and other PACs and lobbyists than any other candidate, Democrat or Republican."

This week's spat is hardly the first time the campaigns have gone negative. In January, they traded dueling radio ads over comments Obama made about President Ronald Reagan. In the run-up to the Nevada caucuses, Clinton attacked (subscription) Obama for his alleged support of the Yucca Mountain nuclear waste facility. And a few weeks later, Clinton hit the air in Wisconsin with a spot criticizing Obama for not holding more debates.