Yep, just 2 years ago Willard "Mitt" Romney was arguing strongly for both an individual mandate to buy health insurance, and also for a "higher tax rate for not having gotten insurance." As Jed Lewison puts it:
The one and only intellectually consistent argument Romney could make against Obamacare is that the difference between Romneycare and Obamacare is that Obamacare was unconstitutional ... but he can't make that argument any more. Deriding it as a tax on everybody or tax penalty on free riders won't work because (a) it's not a tax on everybody and (b) imposing a tax penalty on free riders was his idea.
So no matter how much Republicans might want to call Obamacare a tax, the argument won't fly, especially since their nominee for president is the guy who invented it. For a party that says its top priority is to repeal Obamacare, they really couldn't have picked a worse guy to represent their cause.