In the 1800 election republicans thought the federalists will turn America into a tyrannical monarchy, and the federalists thought the republicans will plunge America into mob rule like the French Revolution. They would have never defended each other.

Things look bad if you have a twenty year time horizon, but they look pretty normal if you zoom out to encompass all of American history.

Re Trump exacting revenge on political opponents, that conduct has endless precedent in American history. (Refusing to concede the election does not; but he was forced out of office nonetheless, which I read as a sign the republic is healthy.)

EDIT: I just remembered about the Aaron Burr conspiracy. Aaron Burr lost the 1800 election to Jefferson by one elector (after over a month of gridlock). He then tried to raise a private army to either march on Washington, or to form his own country and secede from the union. In 1805 Jefferson ordered his arrest and Burr was tried for treason (and acquitted!)