In the course of an article about Democratic candidates' reaction to the capture of Saddam Hussein, Michael Kinsley says
It's a real mystery why politicians find it so hard to admit the obvious about the horse-race aspects of politics. No doubt it requires a dose of blind optimism to be a politician in the first place. [...] But there is also an annoying convention that you must pretend to a confidence you don't feel. Anyone who doesn't realize that this week's news has been a big boost for Bush's reelection is too stupid or blinded to be elected president.
First of all, of course this is no mystery; Kinsley's just having fun avoiding the obvious too. Karl Rove is right about one thing: lots of people vote in elections, intentionally or not, for the guy who they think is the probable winner. If you make it known that you think you're going to lose, you'll lose.
Second, early evidence is that it's a significant boost, but not a big one by the standards of these things: something like 6 or 7 percent in both job approval and reelect numbers. That's less than half the size of the boost Bush got from the beginning of the Iraq war (already evaporated) and about a fifth or sixth the size of the boost he got from the Sept. 11 attacks and the immediate aftermath (also evaporated, much more slowly). Crises and similar big events have transient effects on public opinion, and good news generally doesn't produce the largest boost for the sitting administration; breaking crises do.
Now, given that this is an unusual time right after the breaking news, things are pretty good for him at the moment. Bush's margin over all the Democratic candidates is, right now, at or near 60/40, which usually translates to a landslide win. If the capture of Saddam were an October surprise a week before the election, it alone would probably give Bush an insurmountable lock; but it isn't. As it is, it will help him over the long term if and only if he can parlay the capture into greater stability for Iraq and a good end for the ongoing war there. That remains to be seen.