Massachusetts residents who file a state tax return have to provide proof that they have health insurance. If they can afford insurance but don’t have it, they must “pay a penalty through their tax returns,” according to the state Department of Revenue’s Web site.

This is all thanks to former Gov. Mitt Romney, who set up the system — the best of its kind in the country — and is now trying to pretend he doesn’t remember how it works. On Monday, his campaign said Mr. Romney believed the identical requirement in President Obama’s health care law was a penalty, paid through the tax system. Two days later, Mr. Romney rushed to the cameras to contradict the campaign and insist the mandate was a tax.

Why the switch? As he has on so many issues, Mr. Romney caved to Republican conservatives who want him to campaign on the falsehood that the mandate is a vast tax increase on the middle class. The Supreme Court’s decision that the law is constitutional was disastrous to their cause, so they distorted its basic reasoning. Chief Justice John Roberts Jr. wrote that the mandate is legal under the Congressional taxing power, which Republicans took a step further, saying the mandate must now be a tax. And not just a tax, but a huge, oppressive tax, one of the largest in history.

It is, of course, no such thing. How many “oppressive taxes” are entirely optional? Anyone who does the smart thing and gets health insurance won’t have to pay it. It is, as Mr. Romney himself described it in 2006, a fee to promote “personal responsibility” and prevent healthy people from freeloading. (Among those who won’t be able to comply with the law are poor people living in states where Republican governors refuse to expand their Medicaid programs using federal dollars — though most of those people don’t make enough to have to pay the penalty.)