Health care was a major focus of President Obama's press conference just now. And I'm guessing the write-ups will focus on what he said about the public insurance option.

The idea's advocates will appreciate the strong, feisty defense he offered. The president essentially taunted critics of the idea, most of whom have long insisted the private sector is inherently more superior than the public sector: "If that's the case, this [public insurance option] becomes just one more option. If it's not the case, it's something the American people should know."

Still, Obama refused--first implicitly and then explicitly--to say he'd veto a bill that lacked a public insurance option. This isn't exactly a break with the past. He's generally refused to draw lines in sand on health care, except to say that any reform must control costs while delivering relief to the uninsured and underinsured. Nevertheless, for those who hope reform will emerge without a public plan, this is good news.

But put all of that aside. I thought the most interesting exchange came after ABC's Jake Tapper asked a question about Obama's pledge that nobody would have to give up their health insurance if reform passed. As Tapper explained,

if the government is offering a cheaper health care plan, then lots of employers will want to have their employees covered by that cheaper plan, which will not have to be for-profit, unlike private plans, and may, possibly, benefit from some government subsidies, who knows. And then their employees would be signed up for this public plan, which would violate what you're promising the American people, that they will not have to change health care plans if they like the plan they have.

Tapper's question was more narrow than it had to be. The real issue here isn't simply about a public plan. If private plans made available through the insurance exchange are more financially attractive than the plans employers offer on their own, then you could have the same problem--people fleeing employer-sponsored covearge to opt for these new plans. And it's always possible that if enough people leave an employer's plan, that employer will stop offering it.

The issue is very much on the minds of Capitol HIll reformers. And while there are various ways to mitigate this problem--some better than others--over the long run it's quite possible there'd be some, gradual migration from current employer plans over to plans offered through the exchange. (Again, that's true whether or not there's a public option in the exchange.)

But Obama gave what I considered the proper response, at least on substantive grounds: First, that nobody should have to give up their doctors and hospitals, which is what people care about most; second, that some people are going to lose current coverage because that's how the system works now. Premiums are constantly going up, employers are constantly switching plans. The key is whether government makes that problem worse, either by hastening the shift or dumping people into inferior plans.

...there going to be employers right now, assuming we don't do anything--let's say that we take the advice of some folks who are out there and say, "Oh, this is not the time to do health care. We can't afford it. It's too complicated. Let's take our time," et cetera.

So let's assume that nothing happened. I can guarantee you that there's the possibility for a whole lot of Americans out there that they're not going to end up having the same health care they have. Because what's going to happen is, as costs keep on going up, employers are going to start making decisions. We've got to raise premiums on our employees. In some cases, we can't provide health insurance at all.

And so there are going to be a whole set of changes out there. That's exactly why health reform is so important.

I'll have more to say on this issue later on. First, though, I have a print deadline to meet...

--Jonathan Cohn