Campaigns often feature a division of labor when it comes to speaking about the candidate's opponent, one in which the candidate makes polite but firm criticism, while the surrogates (campaign staff, other elected officials) say much harsher and more personal things. A good campaign makes sure that the two proceed along the same thematic lines so that they reinforce one another, but the fact that the candidate himself is more genteel in his language is supposed to preclude a backlash against him for being too "negative." Frankly, I've always thought this is overblown, particularly the strange custom whereby it's deemed a bit unseemly to refer to your opponent by name, such that saying "Mitt Romney is a jackass" would be horribly uncouth, but saying "My opponent is a jackass" is somehow more acceptable.
As the campaign goes on, this protocol fades away. Candidates' comments take on a harder edge, beginning to resemble the comments their staffs make. It seems we may be entering this phase, as witnessed by this, which Obama said in an interview yesterday:
And the fact that a whole bunch of Republicans in Washington suddenly said, this is a tax—for six years he said it wasn't, and now he has suddenly reversed himself. So the question becomes, are you doing that because of politics? Are you abandoning a principle that you fought for, for six years simply because you're getting pressure for two days from Rush Limbaugh or some critics in Washington?
One of the things that you learn as President is that what you say matters and your principles matter. And sometimes, you've got to fight for things that you believe in and you can't just switch on a dime.
That last part sounds identical to the things George W. Bush said about John Kerry in 2004, and it's more personal than what we usually hear from Obama when he talks about Romney. Obama is talking about this not because a majority of the public agrees with him on the Affordable Care Act (it's a wash) but because it is all but impossible for Mitt Romney to talk about health care without twisting himself into a logical pretzel that reinforces everything people believe about him being a flip-flopper. The personal attack (Mitt Romney is unprincipled) is the point, not the substantive attack (Mitt Romney flip-flopped on the mandate).
The election is in four months, and I think we'll be hearing more and more of these kinds of criticisms from Obama. One of the big reasons is that when David Axelrod says Mitt Romney has no principles, it isn't news, but when Barack Obama says it, it is.