Apparently Reagan was on the ballot in California and won. Those were the days of "favorite son" candidates. Real candidates let local politicians win delegates and use them for bargaining at the convention.
Nobody challenged Reagan, just as nobody challenged the Ohio governor in his state's primaries. The irony is that being on the ballot in California actually gave Reagan more total primary votes than Nixon or any other candidate.
I hope Romney has enough sense not to run again -- or at least not to use the word "redux" if he's crazy enough to run a third time.
Long-time Massachusetts resident John Updike called one of his novels Rabbit Redux
Maybe it's the "red" part of the word that appeals to them, as in Red Sox, or electing Reds to office.