I'm not saying this is a likely scenario, but it's possible. Considering all that's happened over the last couple years we need to question our elected leaders more than ever and hold them to the standards and principles that this country was founded on.
He didn't after 9/11. "Managing" New York was quite properly left to Giuliani and Pataki. There was no curfew declared, only certain areas declared off-limits. And a request for people not to come to work in the city on 9/12.
Your scenario would only sound reasonable to someone who believes that Bush actually wants to cancel the elections rather than be re-elected.
If we did in fact get attacked by dirty bombs 10 days before the election, Bush would win, because people would rally to the President. There would be absolutely no need to cancel the election or do anything rash.
When Bush is put head to head against any of the real democrats who are running, he's won decisively. Bluntly, I have not encountered any current Democratic candidates I'd consider Presidential timber.
Bush has made promises and delivered on them, which is a lot better than Clinton or Bush I did. Bush I probably would have won if he hadn't broken his "No New Taxes" pledge.
I don't think the left has much hope against Dubya, because - love him or hate him - he's been a highly effective leader. Bluntly, that's why your lot hates him. But don't expect him to lose the election unless you can somehow convince the electorate that he's not effective. Unfortunately for you, your last two Presidents (Clinton and Carter) have been notorious for how little meaningful they accomplished in office.
D