A common argument is that it's unconstitutional, and the SCOTUS made a mistake in not saying so.
Many Republicans will use the fallacy of
if one of them Dems say it, then all the rest of 'em think it, reasoning, pointing out what Rep. Phil Hare (D-Illinois) said at a town hall meeting in April 2010: "I don't worry about the Constitution on this, to be honest." ... "I believe it [the Constitution] says 'life, liberty, and the pursuit of happiness.' " Someone pointed out to Hare that's the Declaration of Independence, not the Constitution, he replied, "Doesn't matter to me." So, to many Republicans, Democrats generally do not care about whether the federal legislation they want to pass is constitutionally permissible or not. They argue that the Constitutional power to "regulate commerce among the states" does not mean that Congress can require people to buy health insurance. And again, they think that the Supreme Court made a big boo-boo in claiming that Obamacare is constitutionally permissible because it's a "tax."
Fox News about the town hall meeting.
