WASHINGTON -- President Barack Obama is challenging Mitt Romney's promises to crack down on China's trade practices, saying in an ad released Saturday that the Republican candidate profited by allowing China to strip away U.S. jobs.Obama's ad refers to a recent Washington Post report that several businesses backed by Romney's former private equity firm moved American jobs to China and India to cut costs. In a parting shot, a narrator says Romney is "not the solution. He's the problem."The ad follows Obama's two-day bus tour in Ohio and Pennsylvania, where the president announced plans to file a trade complaint against China at the World Trade Organization for unfairly imposing duties on the exports of U.S.-produced automobiles. Ohio is home to several auto plants and tens of thousands of auto industry workers.Romney has accused Obama of failing to live up to promises to get tough on China.Obama's administration says it has filed seven trade cases against China with the World Trade Organization.The Obama spot is part of a $25 million ad buy in July and will run in New Hampshire, Virginia, North Carolina, Florida, Ohio, Iowa, Colorado and Nevada.