The politics of the issue have changed since Bush and McCain made their proposals, and in a way that strengthens the case for a credit over a deduction. The passage of Obama’s health-care plan means that millions of Americans who now lack insurance will get it.

If Romney comes out for replacing Obama’s plan with a tax deduction, he will be attacked for “taking away” that future coverage. A tax credit, on the other hand, could cover roughly the same number of people. (Here’s a low-end estimate.)

Romney should opt for boldness in covering the uninsured. At the same time, he ought to be cautious about disrupting people’s existing health-care arrangements. That’s the real political vulnerability of both the deduction and the credit: Either one would cause healthy people to leave large company plans for the individual market and thus cause those companies to raise premiums for those left behind.

But there’s a ready solution to this problem: For the time being, let people use the credit in the individual market only if they lack access to a company plan.