On the campaign trail Obama criticized Clinton for forcing people to buy health care insurance without providing the ability to do so and the way to enforce the mandate:

“Senator Clinton is arguing that the only way to get every American covered is if you force every American to buy health care,” Mr. Obama told Iowa reporters in November 2007. “And unfortunately she hasn’t told anybody how she would enforce this mandate.”
[…]
“the only difference between Senator Clinton’s health care plan and mine is that she thinks the problem for people without health care is that nobody has mandated — forced — them to get health care. That’s not what I’m seeing around Nevada. What I see are people who would love to have health care. They — they desperately want it. But the problem is they can’t afford it.”

But now he’s ready to mandate health care:

But during his interview with Diane Sawyer, President Obama said that while “mandates are an example of… something that I was resistant to during the campaign… this is an area where people have made some pretty compelling arguments to me that if we want to have a system that drives down costs for everybody, then we’ve got to have healthier people not opt out of the system. And I think that you have to be careful to make sure that there’s a waiver. So that if we haven’t made health care affordable yet, you’re not punishing people, not only because they can’t afford health care, but — now giving ’em an additional fine.”

He says of his flipflop that his thinking has evolved and that’s what happens when people study this problem deeper. So, I think it’s safe to say that Obama on the campaign trail wasn’t thinking about the problem as deeply as Clinton was. According to Obama, she was more prepared to handle this problem than he was.
The problem is that he thinks that he can lower costs by introducing a public option health insurance. The public option would use public funds at starup but won’t use them once it’s established. The cost of this is pretty expensive and would mean that 23 million people who have insurance now would be dumped into the system (CBO’s estimate).
And I’m not buying it that they won’t receive public funds after start up. Look at the post office and Amtrak. They’re subpar (don’t think so? Take the train to Montreal and then tell what you think) and a black hole for taxpayers’ money. I expect the public option to be more of the same.
BTW, has he solved the problems that he pointed out with Clinton’s mandating of healthcare? How does he enforce it?