If Martial Law were to be declared, indefinatelly. Essentially if such an unprecedented action were to occur, all bets are off. But the check and balance to that are the states themsevles, which tend to be forgotten of as individual nearly-sovreign entities now. They depend on a stable federal government for interstate commerce as well as defence. The states determine what delegates they send to the electoral college to elect the president, (not really the popular vote, of course, and here is another reason why that can be a boon), so essentially the executive branch of government has no say-so to prevent an election.
The Pres is CiC of the Armed Forces, so declaring martial law to forstay an election is by defacto a military coup. Again, in the original United States, before Lincoln turned it into a military dominated hedgemony, the states themselves had a good measure of control over their own soldiers, and an individual's loyalty often lay as much to his state as to the country. (John Brown, for instance, was hung for Treason against the State of Virginia, not against the USA).
I don't think Bush is THAT lousy of a fellow to use the Osama puppet to stave off an election, especially one he has a very good chance of winning.