U.S. President Barack Obama says his government will fight to hold China accountable for practices that harm American auto makers.

Speaking at a campaign stop in the Toledo, Ohio, suburb of Maumee, Mr. Obama said Americans need a “fair playing field.” His administration filed a complaint with the World Trade Organization Thursday that centers on new Chinese duties to American-made cars.

Toledo is home to a big auto plant complex.

Mr. Obama is spending Thursday and Friday on a bus tour through Ohio and Pennsylvania, two crucial states in the November general election.

Mr. Obama carried both states in the 2008 election, and recent opinion polls show him holding a narrow lead over his presumptive Republican opponent, Mitt Romney. The president will use the campaign swing to portray himself as more of a champion of average, working-class Americans than Mr. Romney, a wealthy businessman before entering politics.

The president told supporters in Maumee Thursday that he is fighting for a sense of security in, what he called, the United States' “basic bargain” that hard work pays off and responsibility is rewarded.

Mr. Obama's return to the campaign trail comes days after the U.S. Supreme Court ruled that his landmark heath care reform law was constitutional. The ruling upset Republican opponents of the law, especially the key provision that Americans who do not have health insurance must pay a penalty.

Mr. Romney's campaign initially described the provision, known as the individual mandate, as a penalty, but in a recent interview with CBS News, Mr. Romney himself agreed with other Republicans who called the mandate a tax. Mr. Romney imposed a similar provision in the health care law he enacted during his tenure as governor of Massachusetts.

But the president's campaign tour could be overshadowed by Friday's scheduled release of last month's employment rate. The dismal jobs report from May indicated the economy created just 69,000 jobs, while unemployment rose slightly to 8.2 percent. It was a blow to Mr. Obama's claims that the U.S. economy had improved since he took office.

A report released earlier this week showed that U.S. manufacturing shrank in June for the first time in nearly three years, which raised new questions about the direction of the economy.