Our representatives can't decide to postpone an election, because as soon as they do, they cease to act as representatives, and become agents of a state divorced from its democratic legitimacy. We can't decide, via, say, a referendum after a terror attack, because we'd be voting about whether we're capable of voting rationally, which is incoherent. The only way that makes sense is to vote before a terror attack to grant the government the power to postpone the election briefly. But there are still complications: who decides if a terror attack is serious enough to warrant a postponement? How long can a postponement be? What if there are a series of attacks?
I’m not sure that it is true that as soon as elections are postponed, those doing this become “divorced” from “democratic legitimacy.” What is they decide that elections shall henceforth be held on the second Tuesday of every November instead of the first — is that democratically illegitimate? I don’t think so. Whether the decision becomes illegitimate or not would seem to depend upon the extent of the postponement, and for that Unfogged is raising a very good issue: at some point, a line is crossed and the decision is no longer being made on behalf of the people who are supposed to be represented.