Why would you delay the election? If you can't have one, the States still have the right to name Electors and send them to the College in December, even if you can't have a popular election. I, for one, would prefer you change the date of the Federal election rather than do that, but if someone wants to call the Democrat's bluff...
No really, the Democrats said the same thing in 2002, Bush was trying to delay holding the election. Apparently, we had elections during the Civil War and World War 2, but I don't think those were "democratic" as we would understand them. If I recall, being forced to march through a line of Union Soldiers with a scented McClellan ballot doesn't count as free and fair. I think you should look into delaying the election in the event of a National Emergency, or what would need to be done as one does for potential emergencies, but my guess is, a constitutional amendment would be required, and most state governments have the means to appoint Electors, Representatives and Senators in case of national emergency, which each house of Congress can then accept or reject, which has an interesting history in itself.
Of course, if it's that bad, it's unlikely anyone will vote to change horses in the middle of a river ("A Lame Duck President," as it's known, would leave the US Executive Branch without leadership from early November to mid-January, or latter, depending on the speed of the transition.) which was the outcome for elections held during both the Civil War and the Second World War, and as the election is basically 50/50 right now, Biden would be the one who would will want the delay.
