UPDATE: Drudge points to Obama's assertion, back in September 2009 that the mandate is not a tax.
STEPHANOPOULOS: [I]t's still a tax increase.I have said repeatedly that Obama would be worse off if Obamacare were upheld, but what I'm really seeing is how bad it is for him with the mandate declared a tax.
OBAMA: No. That's not true, George. The — for us to say that you've got to take a responsibility to get health insurance is absolutely not a tax increase. What it's saying is, is that we're not going to have other people carrying your burdens for you anymore than the fact that right now everybody in America, just about, has to get auto insurance. Nobody considers that a tax increase. People say to themselves, that is a fair way to make sure that if you hit my car, that I'm not covering all the costs.
Remember the Democrats got the statute passed by insisting it was not a tax. Now, we learn it is only constitutional because it is a tax. That's got to hurt politically.
ADDED: Romney has at least 3 big arguments:
1. Obama imposed a huge new tax on working people.
2. Obama deceived the American people by saying it was not a tax, when it was.
3. The law made it look like money would go to insurance companies — in the form of new premiums — that would keep premiums low as the companies were required to take on people with pre-existing conditions, but now we find out that the money is really going to go to the federal government. [ADDED: So get ready for your premiums to spiral up and/or for insurance companies to be ruined.]