It's not that Democrats never have fun with Romney's...conviction deficit. But watch President Obama's ads. Listen to him on the stump. Romney's flip flops just aren't a major part of his pitch. That's a deliberate choice. Chicago decided to make this election about contrast, and they don't have a huge incentive to convince voters that Romney might secretly be more liberal than he claimed during the primary.
To me, that's where comparisons between Kerry in 2004 and Romney in 2012 break down. Kerry's political problem with flip flopping was less about inconsistencies in his positions than it was about a concerted GOP effort to paint him as a man without a core.
What Romney's dealing with is entirely different. If anything, Obama is painting Romney as an agent of a true-believing conservative movement who said he'd veto the DREAM Act, wants to privatize Medicare, and should be taken at his word. Romney's running into trouble with conservatives who (probably correctly) don't see him that way at all.
If the right weren't so suspicious of him, he'd have wide berth to appeal to less conservative constituencies. Part of his problem is that the demands of appealing to the GOP base required Romney to take absolutist positions that are basically impossible to walk away from. But a bigger part is that they don't trust him to return to the conservative course if he veers off of it during the general election. And they're erecting barriers on his left so that he can't even try.