That’s why the most important line in his call yesterday to extend the Bush tax cuts for Americans earning less than $250,000 a year was this: “Let's agree to do what we agree on. Right?”

Republicans say they are committed to blocking a tax increase for this group. If members of Congress voted what they say they believe, the one-year extension would sail through both houses.

But of course what the Republicans really care about is extending the tax cuts for the 2 percent of Americans who earn more than $250,000 a year. The GOP loses political leverage if the middle and upper-middle income tax cuts go through because they would then have to pass tax cuts for the wealthy on their own, and polls show that doing so would be unpopular. Obama pressed this point: “Let’s not hold the vast majority of Americans and our entire economy hostage while we debate the merits of another tax cut for the wealthy.”

One of the central skirmishes over the next four months will be over whether Obama takes the blame for Washington gridlock or whether voters place the onus squarely on Republicans in Congress. By challenging Congress “to do what we agree on,” Obama is trying to highlight what is actually going on. This will be one of the most important definitional struggles of the campaign.