From the beginning, advocates of universal health care were troubled by the incompleteness of Barack Obama’s plan, which unlike those of his Democratic rivals wouldn’t cover everyone. But they were willing to cut Mr. Obama slack on the issue, assuming that in the end he would do the right thing.

Now, however, Mr. Obama is claiming that his plan’s weakness is actually a strength. What’s more, he’s doing the same thing in the health care debate he did when claiming that Social Security faces a “crisis” — attacking his rivals by echoing right-wing talking points.

The central question is whether there should be a health insurance “mandate” — a requirement that everyone sign up for health insurance, even if they don’t think they need it. The Edwards and Clinton plans have mandates; the Obama plan has one for children, but not for adults.

Why have a mandate? The whole point of a universal health insurance system is that everyone pays in, even if they’re currently healthy, and in return everyone has insurance coverage if and when they need it.