President Barack Obama promised cheering supporters in Ohio that he would "make no apologies" for his overhaul of health care and mocked rival Mitt Romney's apparent change of heart on his own approach in Massachusetts.

"When you hear all these folks saying, 'Oh, no, no, this is a tax, this is a burden on middle-class families,' let me tell you, we know because the guy I'm running against tried this in Massachusetts and it's working just fine--even though now he denies it," Obama told about 300 supporters at Dobbins Elementary School in the village of Poland.

The president brought up health care often on this week's two day bus tour--the first of this election cycle. On Friday, the president's reelection campaign promoted an interview with an NBC affiliate in Cincinnati in which he hit Romney for changing his tune on whether the individual mandate—the requirement that people have health insurance—is a penalty or a tax. Romney says it's a tax in Obamacare but a penalty in his own plan.

"One of the things that you learn as president is that what you say matters and your principles matter," Obama scolded in the interview. "And sometimes, you've got to fight for things that you believe in and you can't just switch on a dime."

The debate has flared because the Supreme Court upheld Obama's signature domestic policy achievement under Congress's taxing power. Republicans have seized on that to accuse the president of breaking a pledge not to raise taxes on middle-class families. The White House insists that the fine imposed is a penalty, not a tax."We're going to charge you a penalty to make sure that you're not unloading those costs on everybody else," Obama said in Poland. "It will affect less than 1 percent of the population, because most Americans are responsible and do the right thing. I make no apologies for it."

"We're going to keep it moving forward. It was the right thing to do two years ago, it's the right thing to do now, and we're going to keep moving," he said.

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Obama's argument highlighted an interesting aspect of Campaign 2012: While pundits confidently predicted that he would not run on the health care law, which remains unpopular, the president rarely misses a chance to highlight it on the stump.

There is no doubt that the economy remains both the top issue on voters' minds and Obama's greatest vulnerability. But the president has shown no hesitation about making health care an integral part of this appeal to supporters.

Here he is, again, in Poland: "I'm running because I continue to be convinced that in a country like ours, the greatest country on Earth, nobody should go bankrupt just because they get sick. I am proud of the work we did."

Here he is in Sandusky, Ohio, on Thursday: "I'm running because the health care law that we passed was the right thing to do … we fought so hard to make that happen, and now the Supreme Court has ruled. It is time for us to move forward. We don't need to reargue the last two years. I'm willing to work with anybody who wants to make it work, who wants to improve health care in this country and lower costs for individual families. But I don't want to just keep on having political arguments that are based on politics and not on facts."

Here is part of what he said is in Maumee, Ohio, on Thursday: "I'm running because I believe that in America, nobody should go bankrupt because they get sick. I'll work with anybody who wants to work with me to continue to improve our health care system and our health care laws. But the law I passed is here to stay.

And here is a very small sample of what he said in in Parma, Ohio on Thursday: "I couldn't be prouder of the work that we have done in getting this health care law passed."

In Parma, the president met—and embraced—Natoma Canfield, a cancer survivor whose letter about health care inspired him to fight for his overhaul. The letter reportedly hangs in the Oval Office.