Reagan stopped being a Democrat decades before he ran for President, and in the meantime, he had a proven track record as a leading conservative.
As you can see from the link article below - Reagan was elected six times to the Screen Actors Guild. He was a democrat all those years... a union leader.
Reagan LEFT the democrat party in the early 60’s... then became a ‘viable (Republican) candidate in 1964’. The time between ‘early 1960’s and 1964 is NOT that long a time period. Certainly NOT decades.
http://blog.constitutioncenter.org/2013/06/how-ronald-reagan-won-his-first-election/
Reagan had some experience with campaigns in 1966. He had been a union leader as an actor, and he was elected six times to head the Screen Actors Guild.
The former Hollywood actor became active in the Republican Party in California after leaving the Democratic Party in the early 1960s.
Reagan also became a viable political candidate after making a landmark political campaign speech in 1964 on national television supporting the GOP presidential candidate, Barry Goldwater.