Part, but only part, of the issue involves cost.

Journalists have been badgering Warren to get specific about how much taxes would have to go up to pay for Medicare for all. She has, with considerable justice, insisted that this is a bad way to frame the discussion, since any additional taxes would be offset by savings on the huge premiums workers and their employers now pay for private insurance — on average, more than $20,000 a year for a family plan.

The right question is whether the overall costs facing U.S. families would go up or down. Warren has been claiming that for most families, they would go down, but she hasn’t offered specifics. And this vagueness, which has started to seem like evasiveness, is more of a problem for her than it might be for other politicians. As I said, Warren has made policy seriousness a key aspect of her political persona, so her fogginess on health care really stands out.

The plan in the works will presumably try to dispel that fog, but doing so will be tricky. An independent estimate from the Urban Institute (which is, for what it’s worth, left-leaning) suggests that a highly comprehensive Medicare-for-all plan, similar to what Sanders is proposing, would substantially increase overall health spending, although a more modest plan wouldn’t.

But cost isn’t the only issue — in fact, I’m not sure how important it really is, given that full abolition of private insurance remains unlikely in practice. Also, let’s get real: If Warren gets the Democratic nomination, the outcome of the general election isn’t going to hinge on dueling think tank estimates.

The election might, however, hinge on the support of people who have good private coverage and would be nervous about making a leap into the unknown, no matter how many facts and figures Warren deploys.

So what I’ll want to see is whether Warren gives herself and her party enough flexibility to assuage these concerns. I’m not sure what form that flexibility might take. Maybe something like an extended transition period, with greatly enhanced Obamacare (which might actually be politically doable) in the interim?

Whatever Warren comes up with, this is a crucial moment. There are many excellent things in her overall policy agenda; but she won’t get a chance to do those things unless she can extricate herself from what looks like a health policy trap.

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