Kit Seelye's piece in today's New York Times concludes fairly strongly (too strongly, Paul Krugman thinks) that Hillary Clinton's health-care plan won't actually succeed in insuring more people than Barack Obama's, even though hers includes a mandate and his doesn't. Earlier this week Jon Cohn discussed his role in coming up with the 15 million figure (the number of people Clinton claims would remain uninsured under Obama's plan).
Matt Yglesias thinks what's missing from the debate is some sense of who, if anyone, would actually be worse off under Obama's plan than under Hillary's plan. The answer, as I understand it, is "the general public, a little bit, in theory." The key argument in favor of a mandate isn't so much that it provides coverage to everyone (though it's certainly something of a boon to Hillary's campaign, in terms of rhetoric, that she can claim her plan is universal while Obama's isn't). The argument is that a mandate forces people into the coverage pool for whom health insurance would otherwise be a bad deal (mostly young, healthy people), thus spreading risk and lowering the cost of insurance to the sick.
It's a plausible argument and a strong case for a mandate if it's right, but the problem is that I haven't seen any real empirical estimate of the degree to which it would play out this way in reality (though, admittedly, I don't read the literature religiously). There's this Glen Whitman piece from a couple months back, which sparked a fair bit of debate, arguing that a mandate wouldn't, in fact, have that much of an impact on the cost of insurance. And Clinton, so far, hasn't been willing to follow John Edwards's lead in laying out a specific mechanism by which those elusive lucky/healthy folks would be herded into the insurance plans. But it would be helpful (though probably unrealistic) to find some sort of authoritative souce on the question of whether a mandate will really drive down the cost of insurance that much.