Jay might think that a terrorist attack (heaven forfend, etc) would “probably” benefit McCain, but polling on the issue in terms of party* suggests the odds are even — granted, the data are a year old, but it’s not like the Republicans have gotten more popular during the interval. What’s more, if we take the unspeakable off the table, and just ask Americans who they want to handle our many national problems, things trend in the Democrats/Obama’s favor.
*My own caveat is to note that McCain has and continues to poll above “generic Republican.”