Reagan was an extremist. He was just charismatic enough to make it charming. "The scariest words" bullshit really stuck, and he made it seem like the government was the enemy whilst running the government. Trump et al are just the next phase of the same process.
You could run back to the '64 convention and the Goldwater campaign if you want to talk about extremism. But even that sells short how much of a frothing fascist-loving freak Eisenhower ended up proving himself to be by the time he handed the keys over to Kennedy.
If you want to talk about extremism, we can talk about Coolidge and Hoover - men who paved the road to the death of laisse-faire capitalism after presiding over the biggest pump-and-dump scheme yet seen in world history. Hell, Hoover's landslide defeat in '32 was a direct result of his absolute inability to admit he was wrong after a full term in office presiding over the Great Depression.