A nuke goes off in DC and wipes out half of congress, the pres and VP, i think would be cause to delay the elections a bit to have new candidates run for office, prolly with a few month delay.
Many thing can happen that would be reeasonable to delay an election, couple more buildings in NYC toppling for instance.
Having the color code bar be at second to highest level would not be a reason to delay, another theatre of way would not either.
But a massive terrorist attack on US soil would be.