WASHINGTON (Reuters) - Republican presidential candidate Mitt Romney on Wednesday contradicted his campaign's stance on a key part of President Barack Obama's healthcare law, saying its individual insurance mandate amounts to a tax rather than a penalty.

A Romney campaign aide said on Monday that the former Massachusetts governor viewed it as a penalty, a politically sensitive distinction among voters opposed to higher taxes. In reversing course, Romney cited the Supreme Court decision last week.

"Well, the Supreme Court has the final word and their final word is that Obamacare is a tax," Romney said in a CBS television interview. "So it's a tax. They decided it was constitutional. So it is a tax and it's constitutional."

Obama's campaign accused Romney of contradicting both his staff and himself.

When the Supreme Court upheld the healthcare law as constitutional last week, it also declared that the fee charged to most Americans who refuse to buy health insurance amounted to a tax - and not a penalty, as Obama has called it.

On the campaign trail, this was red meat to Republicans who accuse Obama of imposing the biggest tax in U.S. history through his insurance mandate. They maintain it disproves the president's stance that he has not raised taxes on the middle class.

Romney's campaign, however, on Monday in effect undercut his party's assault on the president over healthcare. A campaign spokesman, Eric Fehrnstrom, said Romney did not see the mandate as a tax but as a penalty, just as Obama had said.

Romney rejected, however, any conclusion that a healthcare insurance mandate had been a tax when he implemented one as Massachusetts governor, something Obama has called a model.

The states need not call an individual mandate a tax for it to be constitutional, Romney summarized the Supreme Court as having concluded. "And as a result, Massachusetts' mandate was...a penalty, was described that way by the legislature and by me, and so it stays as it was," he said.

Danny Kaner, an Obama campaign spokesman, accused Romney of contradicting both his own campaign and himself.

"Romney has called the individual mandate he implemented in Massachusetts a tax many times before," he said, supplying a list of quotations going back to 2006.

Romney's remarks on healthcare put him more squarely in line with Republican conservatives who have long feared that he is not truly one of their own.