The question of the “individual mandate,” as the requirement is known, has emerged as one of the most polarizing national political issues of the day. It helped propel the Tea Party movement to mainstream politics, with conservatives calling it a gross overreach of federal power and an infringement on personal liberty.

Mr. Romney’s support of the Massachusetts plan deepened suspicions among many conservatives, who were already wary of him because of the more liberal positions he once took on social issues like abortion and gay rights.

His comments about the mandate being a tax came on an otherwise slow Fourth of July, ensuring that they dominated the news cycle, albeit one that fewer people than usual were paying attention to.

By insisting the mandate is a tax, Mr. Romney has opened himself up to the criticism that he, too, raised taxes as governor. His campaign has sought to portray him as a tax cutter, despite the Obama campaign’s efforts to highlight state fees that rose under Mr. Romney.

In the CBS interview, he insisted that he had not imposed a tax and sought to draw an academic distinction between taxes and penalties.

“The chief justice in his opinion made it very clear that at the state level, states have the power to put in place mandates,” he said. “And as a result, Massachusetts’s mandate was a mandate, was a penalty, was described that way by the legislature and by me, and so it stays as it was.”

Mr. Romney appeared to be making a finer point about the absolute role the Supreme Court plays in setting American law, even if the nuance was lost on many. “Well, the Supreme Court has the final word and their final word is that Obamacare is a tax. So it’s a tax,” he said.