By Kay Henderson

AKNEY, Iowa, Nov 28 (Reuters) - Democratic presidential candidates Hillary Clinton and Barack Obama escalated a dispute over health care coverage on Wednesday as they sought to break free of a three-candidate race in the critical state of Iowa.

Clinton, Obama and John Edwards are deadlocked in the polls in Iowa, which on Jan. 3 holds the first of the state-by-state contests to determine which Republican and Democrat will face off in the Nov. 4, 2008, election.

Clinton, a New York senator who considers Obama her chief rival in Iowa, took aim at his health care plan in an appearance at the Des Moines Area Community College, saying he would leave 15 million Americans without health insurance.

With 47 million Americans without insurance, health care is a major issue in the presidential campaign.

"He's been saying there's no difference between our plans, but his plan would leave at least 15 million Americans uninsured, including more 100,000 people right here in Iowa," she said.

"Who's going to choose who doesn't get covered? ... When I am president there will be no invisible Americans and there will be no Americans without health care," Clinton said.

Clinton, who led a failed effort to change health care coverage in 1993-94 when her husband, Bill Clinton, was president, offered a plan in September that would require all Americans to have health insurance.

Obama, in a conference call with Iowa reporters, said he would make health care more affordable, citing cost as the main reason more people do not have insurance.

"Senator Clinton is arguing that the only way to get every American covered is if you force every American to buy health care. And unfortunately she hasn't told anybody how she would enforce this mandate," the Illinois senator said.

Edwards, a former senator from North Carolina, attacked both their plans, and said that under Obama's proposal, Americans would be able to choose not to buy insurance even though it might be more affordable.

"But it is just as bad to say that everyone will have insurance without a plan to get there. Hillary Clinton says her plan will cover everyone through a 'mandate' but does not provide even the most rudimentary idea much less a detailed plan of how this 'mandate' would work," he said in a statement. (Writing by Steve Holland, editing by Lori Santos and Xavier Briand)

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