If you want to understand what’s really at stake in the battle over Obamacare — as it’s playing out before the Supreme Court and in the presidential race, where the GOP contenders are all vowing to repeal it — you need to watch Mitt Romney’s appearance on Jay Leno last night.

It comes down to this: Should the federal government play any meaningful role at all in helping the millions of uninsured who can’t get coverage — particularly those with preexisting conditions?

On Leno, Romney repeated his vow to transfer health reform back to the states. But he was also repeatedly pressed to say what he would do for those with preexisting conditions if Obamacare were repealed. Without saying how, Romney replied that people with preexisting conditions should continue to get insurance — as long as they’ve been insured in the past. He refused to say what should be done about those who have never had insurance. Here’s the exchange (video here):

LENO: So you would make the law stand for children and people with preexisting conditions.

ROMNEY: People with preexisting conditions — as long as they’ve been insured before, they’re going to continue to have insurance.

LENO: Suppose they were never insured?

ROMNEY: Well, if they’re 45 years old, and they show up, and they say, I want insurance, because I’ve got a heart disease, it’s like, `Hey guys, we can’t play the game like that. You’ve got to get insurance when you’re well, and if you get ill, then you’re going to be covered.’

LENO: I know guys at work in the auto industry, and they’re just not covered...they’ve just never been able to get insurance. And then they get to e 30, 35, and were never able to get insurance before. Now they have it. That seems like a good thing.

ROMNEY: We’ll look at a circumstance where someone was ill, and hasn’t been insured so far. But people who have had the chance to be insured — if yu’re working in an auto business for instance, the companies carry insurance, they insure all their employees — you look at the circumstances that exist. But people who have done their best to get insured, are going to be able to be covered. But you don’t want everyone saying, `I’m going to sit back until I get sick and then go buy insurance.’ That doesn’t make sense. But you have to find rules that get people in that are playing by the rules.

That last section is a bit garbled, but the basic fact here is that when asked what the federal government should do about those with preexisting conditions who have never had insurance, Romney won’t say. This echoes Romney’s recent exchange with a student who asked what the federal government would do to help with student loan debt. Answer: Nothing.

What’s particularly interesting about the above exchange is that Romney himself detailed exactly the problem that the individual mandate is designed to fix: If people wait until they get sick before getting insurance, it fouls up the system. As he puts it, this “you’ve got to get insurance when you’re well.” Romney’s recognition of the policy problem, of course, is why he passed a mandate at the state level in Massachusetts.

But now Romney is obliged by GOP primary politics (and perhaps by his actual beliefs) to insist that a federal mandate is an unconstitutional usurpation of American freedom. So he’s forced to give a nonsensical answer to the core policy and moral question that’s left behind if we do away with Obamacare: What should the federal government do about those who can’t get insurance covarge, thanks to preexisting conditions?

Until Romney details otherwise, his answer, for all practical purposes, is: Nothing.

Update: The Obama campaign responds:

“Only in Mitt Romney’s world of tax cuts for billionaires and elevators for his cars would denying health care coverage to Americans with pre-existing conditions make sense. But to most Americans, ensuring insurance companies don’t discriminate against people with pre-existing conditions means the difference between solvency and bankruptcy and life and death. With every day of this campaign, Mitt Romney shows why Americans can’t afford his out-of-touch and extreme policies.”

* The choice we face: Jonathan Chait, writing about similar comments from Mitch McConnell, boils it down:

The choice we face is not between Obamacare and some different, even more “market-friendly” alternative reform. It’s between Obamacare and subjecting millions of Americans to the insecurity and suffering of lacking health insurance. The uninsured can have the Republicans’ answer now. Their offer is this: nothing.

* What happens to health law if mandate is struck down? With a third day of Supreme Court arguments set for today over what would happen if the mandate is ruled unconstitutional, N.C. Aizenman runs through the various possibilities, with some experts concluding that if the mandate goes, the whole law will collapse with it.

* Supreme Court won’t save GOP on health care: Relatedly, Taegan Goddard points us to an important point from David Frum:

Republicans will need a Plan B. Unfortunately, they wasted the past three years that might have developed one. If the Supreme Court doesn’t rescue them from themselves, they’ll be heading into this election season arguing, in effect, Our plan is to take away the government-mandated insurance of millions of people under age 65, and replace it with nothing. And we’re doing this so as to better protect the government-mandated insurance of people over 65 — until we begin to phase out that insurance, too, for everybody now under 55.

Exhibit A: The Romney exchange above.

* Why the mandate is constitutional: The New York Times editorial board makes a strong case that if the Supreme Court strikes down the mandate, it will be overstepping its own powers:

Congress has indisputable authority to regulate national markets and provide for the general welfare through its broad power to tax. Nothing about the mandate falls outside those clearly delineated powers...

If the Supreme Court hews to established law, the only question it must answer in this case is modest: Did Congress have a rational basis for concluding that the economic effects of a broken health care system warranted a national solution? The answer is incontrovertibly yes.

* Could SCOTUS loss help Obama? I’m skeptical, but Ross Douthat makes the case, arguing that the 2010 elections issued a brushback pitch to liberalism that made voters more inclined to give Obama a second chance, and that the same could happen again:

Stripping away the law’s most unpopular component might make the rest of it marginally more popular. And setting a clear limit on liberalism’s ability to micromanage Americans’ private decisions might make voters feel more comfortable voting to re-elect their micromanager-in-chief.

The question, though, is whether a defeat for the mandate reinforces the narrative that Obama squandered time and energy on an ideologically driven but fruitless Big Government health care overhaul when he should have focused on the economy.

* Obamacare question of the day: Jill Lawrence asks a good one: If the mandate is struck down, “in practical terms, would the country really know what it has lost?”

* Romney’s negatives jump: A new Washington Post/ABC News poll finds that half of all registered voters now view Romney negatively, a sign of the toll the primary has taken on him. The question, however, is whether the dynamic will undergo a fundamental shift once Romney’s the nominee, giving him a chance to reintroduce himself to core swing constituencies who will then give him a second look.

Also: Obama’s favorable rating is up to 53 percent.

* Can Romney Etch-A-Sketch away his problems? His numbers among independents and moderates are absolutely awful, though, again, he’ll have another chance to introduce himself to them on his own terms.

* Obama leading Romney in key swing states: New Quinnipiac polling finds that Obama has opened up a lead over Romney in three key swing states — he’s ahead 49-42 in Florida; 47-41 in Ohio; and 45-42 (which is statistically insignificant) in Pennsylvania. However, his approval numbers are lagging in all three.

Key finding: Although large majorities think the economy is in recession, roughly six in 10 voters in the three states think it’s starting to improve — and as always, what matters are perceptions of its direction.

* Dems to hit Paul Ryan over Medicare vote: In advance of today’s vote on Ryancare 2.0, the DCCC is going up with a new billboard in Ryan’s hometown of Janesville, Wisconsin, proclaiming that he is “protecting millionaires instead of Medicare.”

Republicans hope to invoke Senator Ron Wyden in order to call this year’s plan “bipartisan” and escape last year’s debacle, but new polling shows overwhelming public support for keeping traditional Medicare as it is, versus only one forth of Americans supporting the new plan.

* And no end to the dissembling: Glenn Kessler does a nice job taking apart Romney’s chest-thumping about Russia being our leading “geopolitical foe.”

What else?