The McCain health-care insurance tax credit may well be one of the most misunderstood proposals of this presidential election. Barack Obama has been ruthless in his attacks. But the tax credit is highly progressive and will provide a powerful incentive for people to purchase health insurance. These features under normal circumstances should endear Democrats to the proposal.

There has been a lot of rhetoric and misstatements, but what exactly does Sen. McCain have in mind? He would replace the current income tax exclusion for...