OK, it's not the most important sentence you will read today. But it's certainly the most revealing if you want to understand how the dynamics of the presidential race may be changing.

It comes via a press release from the McCain campaign, out just minutes ago. Here it is, right at the top:  

ARLINGTON, VA -- Today across America, members of the Small Business Leaders for McCain coalition came together to talk about John McCain's plan to grow jobs and to respond to attacks from Democrats, who have resorted to distorting John McCain's words to obscure the fact that they have no economic plan of their own.

So, you say, what's the big deal? The big deal is that phrase "respond to attacks from Democrats."

For the last two weeks, Obama has been the one responding to attacks from the McCain campaign--on everything from the ludicrous charge that Obama wants to teach five-year-olds about sex to the demonstrably false suggestion that Obama would raise taxes on the middle class (even though I wish he would!).

Sometimes Obama has responded artfully to these attacks, sometimes he hasn't. But, either way, he's been letting McCain define the agenda. He's been fighting on McCain's turf. And that's no way to win an election.

Today, clearly, McCain is the one on the defensive. And while the news from the real world has more than a little to do with this, so does the Obama campaign's decision--made before the weekend, by all appearances--to go hard after McCain this week on economic policy.

It's working. And I hope the Obama campaign keeps it up. Among other things, they happen to be right on the merits.

Now if they'd only cut an ad or two on health care, I'd be really happy. 

--Jonathan Cohn