Norm Ornstein in the Atlantic:

The Constitution leaves a lot of leeway for Congress. So it is time to consider a new law, one that cleans up the issues and discrepancies in the existing succession act but does more. It should allow for a special election for president and vice president under extraordinary circumstances. Those could include a terrorist attack or an attack by a foreign power or others on Election Day or on the election system or process that destroys or distorts the results. It can also include foreign interference in the election combined with a winning party’s involvement in or reinforcement of the interference. Such a provision would have to be carefully drawn and set a high bar, to avoid any chicanery to call an election for the wrong reasons. It would probably have to require a two-thirds vote in both houses of Congress.