Kevin Drum explained long ago a core problem in merely ending the insurance company ban on coverage of people with pre-existing conditions. Drum wrote then that Obama hoped to maneuver Republicans into precisely this position.
For Obama, this is the ballgame. My guess is that he wants to maneuver Republicans into either (a) admitting that they're unwilling to regulate this, which would be highly unpopular, or (b) admitting, however grudgingly, that the practice needs to be banned. Because if they admit it has to be banned he can make the following argument:
If insurance companies are forced to take on all comers, then people can game the system by buying insurance only when they get sick. This would obviously decimate the private insurance industry.
So you have to require everyone to buy insurance at all times. It's the only way to have a broad pool that keeps costs down (another frequent Obama talking point.)
But obviously you can't force poor people to buy insurance they flatly can't afford. So if you mandate coverage, then you have to subsidize low-income families that can't afford insurance, and you have to provide incentives for small businesses so that they can cover their employees.
And if you do that, you have to have a funding source. Preferably one that also helps rein in premium costs. Like, oh, an excise tax.
Romney's magic wand and Paul Ryan's amazing secret plan, to be revealed only AFTER the election, are only so much more snake oil.
Oh, and by the way, Romney really hasn't changed anything about his position on health care. If you already have insurance, you're OK. Otherwise ....
UPDATE: Romney's pre-existing condition coverage would OMIT 89 million Americans. No rich ones, you may be sure.
Romney's tax plan is similarly problematic.