Good question
Michael Tomasky from the "American Prospect" has an interesting strategy for Kerry: attack on the Osama Bin Laden issue. He also answers the question, "If you do so, what happens if Bin Laden is captured before the election?", with this answer, "If he's captured, Bush wins anyway."

Here's what he says:

"Now, beginning with this Thursday's debate, Kerry should strike right at the dark heart of Bush's national-security failures. Where, he should ask, is Osama bin Laden? We sent about 12,000 troops to Afghanistan. We removed the Taliban, but the man who orchestrated the September 11 attacks and then delivered to the world a videotape gloating about them slipped away. Then -- boom -- we sent 130,000 troops to Iraq, which was somehow more important than getting the man who killed 2,700 Americans. Bin Laden still circulates.

Can you imagine the furor from the Limbaugh/FOX News corner if a President Gore hadn't captured the 9-11 malefactor by now (and had diverted resources to go after someone else instead? And how about if -- remember this one? -- anthrax packets had been sent to congressional Republican leaders and Gore's Justice Department hadn't yet nabbed a culprit?)? There should be blind outrage afoot in this country that bin Laden hasn't been captured or killed. And Kerry should stoke it.

I can already hear the nervous Democratic operatives: Ooh, that's too risky. What if we capture bin Laden between now and November 2? The argument is cut out from underneath Kerry. Well, that's inarguable. But guess what? If we capture bin Laden between now and election day, Bush wins anyway, no matter how you slice it and no matter what Kerry did or did not say. So the risk involved in talking directly and aggressively about bin Laden -- Kerry began to do so at Temple University last Friday, but the invocations weren't at the center of that speech -- is in fact rather minimal. From the Kerry campaign's point of view, the possible bad political outcome (Bush captures bin Laden, wins election) would have happened anyway, while the possible good political outcome (nation finally focuses on why this man is still at large, Bush put on defensive) will come only if Kerry starts asking the question."