But at the heart of the dispute is something much larger—the growing liberal concern over what might be called the Obamacare Nightmare Scenario: that too few people, who are too sick, will sign up for coverage under the law, that premiums will rise in the exchanges, and that this will reinforce public skepticism of the law as an unworkable burden whose primary effect is to cause costs to rise. …

They’re right to worry. In part because, as Emanuel notes in his piece, this sort of enrollment push has never been tried at this scale. But also because a version of what they worry about—low enrollment, an unusually sick population, and spiraling costs—has happened before, in Obamacare’s first, smaller-scale attempt to expand coverage to the uninsured. …

That concern was half right. Somewhere between 350,000 and 400,000 people were expected to enroll in the program. Instead, just 135,000 signed up—and then only after the administration went on an aggressive enrollment push. Yet even as The New York Times reported this week, even with far lower than expected enrollment, the cost of claims in the program has “far exceeded White House estimates, exhausting most of the $5 billion” the legislation provided to fund the program.