In fact, health care experts say, it’s not out of the question that the Obama administration could face the worst-case scenario on Jan. 1: the number of uninsured Americans actually goes up.

That’s a long shot, and there are plenty of reasons why it might not happen, since there are other ways those people could replace their health coverage, like signing up directly with insurers. Not all of the policies will expire in December. And even if the ranks of the uninsured did increase, it could be such a brief event that no one would ever be able to confirm it.

But even with all the variables, one thing is for certain: the Obama administration has one seriously long road to travel from the signups it has now to the number who will likely need to replace their coverage. That’s a bad place to be, given that the point of the law was to cover more people, not fewer people.

“It’s hard to know [the outcome], but it’s hard to see how it’s not a heavy lift given the numbers,” said Gail Wilensky, who ran the Medicare agency under the first President George Bush.