This political reality makes the tax deal a bad bargain for Democrats. Think of it this way: The deal essentially sets up 2011-2012 to be a repeat of 2009-2010. Once again, there would be initial benefits from the stimulus, and decent growth a year before the election. But as the stimulus faded, growth would tend to stall — and this stall would, once again, come in the months leading up to the election, with seriously negative consequences for Mr. Obama and his party.

You may say that economic policy shouldn’t be affected by partisan considerations. But even if you believe that — how’s the weather on your planet? — you have to consider the situation likely to prevail a year from now, as the good parts of the Obama-McConnell deal are about to expire. Wouldn’t there be pressure on Democrats to offer Republicans something, anything, to improve economic prospects for 2012? And wouldn’t that be a recipe for another bad deal?

Surely the answer to both questions is yes. And that means that Mr. Obama is, as I said, paying for the release of some hostages — getting an extension of unemployment benefits and some more stimulus — by giving Republicans new hostages, which they may well use to make new, destructive demands a year from now.