>California was, in Presidential politics, a Republican stronghold, not a swing state, prior to 1992. (Between Truman in 1948 and 1992, the only Democrat to win in California was Johnson in 1964; precious little swinging going on for a “swing” state.)

Look at the margins of those wins during those years (https://en.wikipedia.org/wiki/United_States_presidential_ele...). The state consistenly voted very close to how the nation voted. Some years it voted slightly more Republican (Nixon in 1960 and Ford in 1976), other years less (Eisenhower won fewer Californian votes in 1956 than in 1952, contrary to the national trend), but it was always close when the national race was close. That Michigan, Pennsyvania, and Wisconsin hadn't voted Republican in 30 years didn't not make them swing states in 2016.