Three years later, a 1974 storyline found the Sentinel of Liberty battling an elusive evil organization named the Secret Empire, and simultaneously being tarnished by a smear campaign against him engineered by C.R.A.P., "the Committee to Regain America’s Principles,” run by a thinly disguised parody of then Chief of Staff H.R. Haldeman.
After several issues, Captain America defeats the organization and in July 1974’s issue #143 unmasks their leader—then-President of the United States, Richard Milhous Nixon.
Note that this was not a clone, not a robot, not a shape-changing alien imposter or mirror-universe doppleganger. This storyline suggested that the sitting President was a supervillain seeking the overthrow of the United States.
At the height of the confrontation Nixon commits suicide rather than face arrest. He is replaced by a double, and the Watergate scandal is engineered to allow the imposter to “resign” rather than let the public know how close America came to Armegeddon.
Digusted by the whole affair, Captain America abandons his superhero identity for a time. Comics being comics, he rejoins the fray later the next year.
--And through it all, Marvel felt no need to offer commentary, clarification, or apology.