This year, however, there’s a surprising addition: the health-policy primary.

Given the subject’s legendary political toxicity (many give it a starring role in the devastating losses Democrats suffered in 1994) and eye-glazing density, it’s rather surprising to watch the speed and eagerness with which the candidates are unveiling ambitious, substantive and heavily progressive plans. John Edwards, Barack Obama and Bill Richardson have all brought out their full proposals. Hillary Clinton has outlined the cost-control measures in her proposal, with the actual details of the coverage portion presumably on the way.

But like so many of the primaries, this one is not about its putative subject. The Shrum primary, for instance, wasn’t about the utility of Bob Shrum’s legendarily ineffective advice, but about the imprimatur of establishment support he offered. The money primary isn’t fully about the money—primaries, particularly those in Iowa and New Hampshire, really don’t cost that much, and only occasionally reward the flushest competitors—but about the depths of elite support the fund-raising signals.

Similarly, the health-policy primary, and its attendant attention to policy minutiae, is about more than subsidy levels and mandate schemes: It’s a demonstration of how each of the candidates would govern, and how each would deal with the resuscitated progressive movement, which has settled on universal health-care reform as its chief domestic priority.

The health-policy primary has been particularly revealing about Mr. Obama and Mr. Edwards, both of whom have used detailed policy proposals to demonstrate courage and commitment to progressive ideals that aren’t apparent from their histories.

Mr. Edwards was the first out of the gate, with an expansive, detailed and astonishingly ambitious health-care-reform proposal. Portions of it—notably the creation of a publicly run insurance option—would have been considered unthinkably far left even four years ago. The contrast was particularly stark in the context of Mr. Edwards’ campaign in 2004, when he ran as a sunny Clintonian centrist.

His proposal this year was meant to explode any lingering hunches that he might be running the same campaign. Without a fundamentally progressive record to fall back on, he needed progressive policies to prove his transformation: By running on public commitments to transformative change, he’s ensuring the left that they—and not his elite backers from four years ago—are his core support, and the constituents he’ll seek to serve. He’s also signaling a belief that reform will not be achieved by soliciting the insurance industry’s cooperation, but by bringing about its defeat.

Mr. Obama, by contrast, released a proposal meant to underscore his attributes as a progressive unifier. Like Mr. Edwards, he has proposed a public coverage program, and subsidies for the poor, and a vision of universality. But this is a softer, gentler form of progressivism: There is no mandate demanding that everyone gets covered, and little evidence that the public option will be taken advantage of. The plan is meant to reassure the left of his basic sympathy to their view while sanding the edges that could make him seem combative and radical. It’s liberalism without the bite.

Mr. Richardson, for his part, offered a vision that’s not terribly liberal or transformative. It’s not even universal. It’s part and parcel of his claim to be a doer, rather than a dreamer.

And finally, there is Hillary Clinton. Unlike the others, she has not released a full plan. Unlike the others, she scarcely needs to.

Mrs. Clinton has a record on this—a demonstrated exhibition of her commitments. A plan would leave her seeming no more, or less, engaged with the issue. Indeed, the question with her isn’t political, it’s personal. No one knows how she really feels about health-care reform after the searing experience in 1993 and 1994. She speaks of her battle scars, but whether they would serve as reminders when she re-enters the fray, or warnings to do nothing of the kind, remains a mystery.

Each of these candidates has used health care as a definitional domestic issue to signal their approach to governance. Mr. Edwards will govern from the left, believing the country ready for a sharp turn toward progressivism. Mr. Obama will sympathize with the left, but will seek consensus and stakeholder buy-in and make many of the compromises necessary to achieve wide participation. Mr. Richardson will be an executive, focusing on making things better, but not shooting for the best. And Mrs. Clinton is battle-hardened, and will approach the issue with neither the naïveté nor enthusiasm of political youth.

These are all distinct, and plausible, visions of executive conduct. And they offer something that the Shrum primary, the money primary, the invisible primary, and indeed the actual primaries, don’t: A window into how the candidates will govern, rather than just how they will campaign.

Joe Conason is on paternity leave and will return to this space next week.