Only if the Constitution is annulled due to war or some major disturbance can elections be canceled. In that case, the President is military dictator. He can hold elections again if the Constitution is ever restored.
Note that LBJ asked the NSC to explore this scenario in 1968 after the unpleasantness at the Democratic Convention in Chicago. He feared that the antiwar youth and Black Panthers might join forces and disrupt the elections via terrorism.
Federal elections can’t be canceled as you think. The terms of office for Congress and the President require elections so that the offices can be filled. Without elections, the offices expire, and no one fills them.
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I didn’t think they could be cancelled at all, but it looks as though they can be if Congress is the only body who has the authority to conduct a Presidential election and then it simply fails to hold an election. I’ve been trying to gauge the likelighood of Congress doing the unthinkable and It’s looking more likely ... unless Devolution has been executed.