Obama has won 10 consecutive state nominating contests since February 5. The string of victories has put him ahead in the race for delegates to a nominating convention this summer where the party will pick a candidate for the November election.

Many analysts say Clinton must win contests in the delegate-rich states of Ohio and Texas on March 4 to cut Obama's lead and still have a chance at the nomination.

Campaigning in Ohio, Obama told a roundtable on health care at a hospital in Columbus that his health care plan would cut medical costs more than hers. He also touched on the issue that Clinton had complained about.

"The main difference between us is that Senator Clinton includes a mandate, which means she'd have the government force you to buy health insurance, and she said that she'd consider 'going after your wages' if you don't," Obama said, adding that he disagreed with that approach.

Meanwhile, Clinton said the campaign leaflet on health care reminded her of health insurance industry attacks on her plan. She also said another leaflet Obama's campaign issued misrepresented her views on trade agreements such as the North American Free Trade Agreement, or NAFTA.