I don't think that having a contingency plan is totally out of the question, but the context in which the administration seems to be bringing up the subject is suspect.
Perhaps define affect?
The 9/11 attacks drectly affected 3,400 voters, in that they perished in that attack. It also indirectly affected the psyche of NYC and the country, but not so much that the country ground to a literal halt (air travel excepted). If the attack had happened on 11/2 in a presidential election year, rather than 9/11, the country would have gone on with the elections, although, like Spain, the outcome of the voting may have been altered.
The scope of a terrorist attack, even in a heavily populated metro area, would have to be truly massive to physically prevent people from reaching the polls, like something on a nuclear scale. So what is the criteria that would need to be met in order to postpone an election?
After all, a presidential election was held in 1812, after war had been declared in June of that year, U.S. forces had lost a number of battles against British-Canadian forces, and british troops were on American territorial soil.