In The Nation, Greg Mitchell says that Ronald Reagan’s shift to the GOP started with the 1950 California Senate race between Richard Nixon and Helen Gahagan Douglas.  Reagan initially backed Douglas, Mitchell says, but then moved in RN’s direction.  He writes:

One night, not long before election day, actor Robert Cummings received a phone call from Reagan (he wrote in a memoir). Cummings had Republican leanings but not strong ones, and had his own political disputes with Reagan when they acted in King’s Row in 1942. Cummings considered Reagan one of the most honorable men in town and joked that he ought to run for president some day.

Cummings would re-create that phone conversation this way:

“I’m trying to help a senator get elected and we’re giving a party for him tomorrow night,” Reagan reportedly said. “Can you come?”

“Who is the senator?”

“His name is Richard Nixon.”

“But isn’t he a Republican?”

“I’ve switched,” Reagan announced, according to this account. “I sat down and made a list of the people I know, and the most admired people I know are Republicans.”

Of course, Cummings may have imagined this; no evidence of Reagan hosting a party has turned up. But his lack of offering further help, or money or any support for Douglas in the fall campaign — despite efforts to secure any of it — supports the view that indeed he did back Nixon on election day.