Mitt Romney came under fire this week for saying that Obamacare’s fine for not buying health insurance is a tax, not a penalty — the opposite of what he said last week after the Supreme Court ruling came down.

One possibly big reason for the semantic woes of the GOP nominee-to-be: It’s no longer so easy for opponents of Obamacare to call the law a tax or unconstitutional. In fact, with one exception, anyone who calls Obamacare’s fine a tax, citing the Supreme Court, must admit that it is constitutional, per that same court ruling. That’s true for Romney and every other Republican now insisting that Obamacare’s fine is a tax hike.

“The Supreme Court has the final word,” Romney said on Wednesday, “and their final word is that Obamacare is a tax. So it’s a tax. They decided it was constitutional. So it is a tax and it’s constitutional.”

Some conservatives will try to have their cake and eat it, too — to claim that Obamacare’s fine is both a tax and unconstitutional. But in light of the court’s ruling, the claim demands some careful thinking. Conservatives must argue that the underlying policy can be thought of as a tax, which accepts the opinion of the court’s majority. Then they must argue that even the government’s constitutional taxing authority doesn’t authorize the fine, a claim the majority rejected.

Aside from being an ambitious position, that’s also not a very comfortable spot for Romney. He has said he agrees with the content of the dissent to the ruling, which rejected the notion that Obamacare’s fine is a tax. But he said he will call it a tax anyway because the majority’s word is “the law of the land.” He can’t hold both positions and quibble with the majority’s decision in order to justify labeling the fine an unconstitutional tax.

If the unconstitutional tax line becomes conservative orthodoxy, though, Romney might once again feel pressure to alter his rhetoric, even if that makes the candidate’s position on Obamacare far more incoherent.