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

Obama zeroed in on the individual mandate, the requirement that nearly all Americans buy health insurance or pay a fine.

The Supreme Court upheld the mandate last week, citing Congress' taxing powers -- and leading Romney and other Republicans to accuse Obama of being a tax raiser via his health care law.

"On the health care bill, Mr. Romney was one of the biggest promoters of the individual mandate," Obama said. "In Massachusetts, his whole idea was that we shouldn't have people who can afford to get health insurance to not buy it and then force you or me, or John Q. Public to have to pay for him when he gets sick. That's irresponsible. That's exactly what's included as part of my health care plan."

He added:

"And the fact that a whole bunch of Republicans in Washington suddenly said, this is a tax -- for six years he said it wasn't, and now he has suddenly reversed himself. So the question becomes, are you doing that because of politics? Are you abandoning a principle that you fought for, for six years simply because you're getting pressure for two days from Rush Limbaugh or some critics in Washington?"

The Obama campaign released a transcript of his remarks.

Romney has said his health care plan works for the state of Massachusetts, but Obama's version is too expensive and too bureaucratic.

The Republican has vowed to repeal what he calls "Obamacare" if he gets elected.

Responding to Obama's latest comments, Romney spokesperson Amanda Henneberg said the president has been trying to hide the fact that his health care plan involves a tax.

Henneberg's statement:

"President Obama is right -- as president, what you say matters.

It matters that President Obama told the American people that Obamacare was not a new tax, then sent his lawyers to convince the Supreme Court that it was a new tax, and now is insisting -- again -- that it is not a tax.

Americans deserve straight answers from their president. Mitt Romney believes Obamacare is a job-killing law that raises taxes, cuts Medicare, and gets between patients and their doctors.

In order to repeal Obamacare, we must replace President Obama."

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