"His plan would take Medicare and Medicaid and the Children's Health Insurance Program and the Affordable Care Act health-care insurance and private employer health insurance and he would take that all together and send health insurance to the states, turning over your and my health insurance to governors," Clinton said, naming the state's Republican Gov. Terry Branstad. "I don’t believe number one we should be starting over. We had enough of a fight to get to the Affordable Care Act. So I don’t want to rip it up and start over."
Clinton called Sanders's plan a "risky deal."
Sanders has proposed a "Medicare-for-all" health care system -- or a universal, single-payer health care plan -- that he says would mimic European health-care systems that guarantee health care to everyone.
Clinton said, as she has in the past, that Sanders's plans would cost up to $20 trillion and would increase the deficit and require higher taxes on the middle class. Clinton has pledged not to increase taxes on the middle class.
"I want to be an aggressive Democrat who gets things done," Clinton said. "I want to do it in a responsible way, paid for in ways we can afford. "
Sanders spokesman Michael Briggs said that while Sanders advocates a single-payer system, he has not developed legislation as a presidential candidate to implement it. Under a 2013 single-payer bill introduced by Sanders, Briggs said, it was "national legislation for all our states."
"Secretary Clinton is inaccurate in suggesting that Republican governors would be able to circumvent the law and deny implementation in their states," Briggs said.
Sanders has also argued that under a single-payer approach, the typical family would save thousands of dollars a year because they would no longer have deductibles or co-payments.
Firing back after Clinton's speech, Sanders' Twitter account highlighted a 2013 tweet from Clinton's current campaign chairman John Podesta in which he noted that he had signed up for Medicare online in 5 minutes. "Single payer anyone?" Podesta asked.
The Sanders tweet said: "If you ever want to work for a campaign that shares your values on health care, there's always room at Bernie 2016."
The race between Sanders and Clinton has been heating up in both Iowa and New Hampshire. The most recent NBC News/Wall Street Journal/Marist poll showed Sanders within 3 percentage points of Clinton -- the closest the race has ever been in any public polling. And recent polls have shown Sanders leading in New Hampshire, a neighbor to his home state of Vermont.
Meanwhile, the Clinton campaign has launched aggressive, surgical attacks on Sanders -- particularly on his gun record. In Iowa, she said that Sanders voted to give gun makers immunity while she and President Obama, when they were in the Senate, voted against it.
"I think it's time for us to have the kind of spirited debate, that you would wish us to have," Clinton said. "We do have differences and you deserve to know what those differences are."
John Wagner contributed to this report.