Last Thursday, the Supreme Court upheld the vast majority of the healthcare law, including the individual mandate to buy insurance, declaring it constitutional under Congress's authority to levy taxes.
Shortly after the decision, Republicans began citing it to argue that the healthcare law amounts to an enormous tax increase.
"The Supreme Court saved ObamaCare but at the expense of exposing it as simply a massive collection of taxes — 20 taxes at least," Norquist wrote.
The anti-tax activist also slammed Chief Justice John Roberts, who wrote the majority opinion and has become a pariah among conservatives in the decision's wake, though he was appointed by President George W. Bush.
Because of Roberts, Norquist wrote, "the federal government can do anything — anything at all — if it goes through the nicety of using the bludgeon of a tax hike to force you to submit. Do this — or we tax you $1,000.
"Do this or the government takes $10,000. Do this or the government takes everything you have. All perfectly constitutional, according to Justice Roberts," he wrote.
The arguments about the mandate being a tax were undermined earlier this week when a senior Romney adviser said in an interview that his boss does not see the mandate as a tax.
"The governor disagreed with the ruling of the court. He agreed with the dissent that was written by Justice [Antonin] Scalia that very clearly stated that the mandate was not a tax," Eric Fehrnstrom said.
But that position was quickly reversed by Romney, who said on Wednesday that the mandate "is a tax and it's unconstitutional."
On the campaign trail Thursday, the Obama campaign slammed Romney and accused him of flip-flopping.
White House spokeswoman Jen Psaki said that Romney has long defended his individual mandate in Massachusetts as a "penalty," not a tax.
"The president agreed with him," she said.
"It is clear that [Romney] is being impacted by the push from the right, the Rush Limbaughs of the world, congressional Republicans, pushing him to go back on a decision and a defense that he's had in place for years."