Unless they're waiting for just before the election or during the conventions.
Sure, leave the nukes lying around for months...
Hell, the explosives are likely to decompose enough that they don't go off.
If they are doing that, then they are idiots. Taking a risk on being caught just so that they can wait for a convention? Ridiculous. Just detonating it would be a major victory for them. They can't even blow off a conventional bomb, though.
Mind you, I am not suggesting that we should not be worried about the terrorists getting the a-bomb. It' just the notion that they have got bombs here and are waiting for the right moment that I find ridiculous.