Democrats Hillary Clinton and Barack Obama sparred with each other over negative campaigning, health care and free trade Tuesday, a week before primaries in Texas and Ohio could either effectively seal the nomination for Obama or throw the contest wide open again with a strong Clinton performance.

Those contests are must-wins for Clinton if she is to continue to contest Obama for the nomination, as even former President Clinton suggests.

Debating at Cleveland State University, Clinton repeated angry claims from the campaign trail that Obama mischaracterized her stances on health care and NAFTA in political material mailed to voters in Ohio.

"I have a great deal of respect for Sen. Obama, but we have differences," she said. "In the last several days, some of those differences in tactics and choices that Sen. Obama's campaign has made regarding fliers and mailers and other information that has been put out ... have been very disturbing to me."

The mailers, which Obama defends, claim that Clinton's health care plan would force people who don't want insurance to buy it. They also say she has been inconsistent on NAFTA, which many in industrial states like Ohio blame for shipping blue-collar jobs overseas.