Eliana Johnson reports for National Review Online on a California presidential primary that will mean more to Republicans than any others in the past 50 years.

What, exactly, does a California Republican look like? The question is now a subject of heated debate, and the answer may very well determine the outcome of this historic GOP primary.

California hasn’t played a decisive role in choosing the Republican nominee since 1964, when it broke for Barry Goldwater over Nelson Rockefeller in a contest so close that Rockefeller went on to protest the results at the convention in San Francisco. This year, California’s Republican voters are set to make an equally momentous decision: The state’s 172 delegates will either put Donald Trump over the top and deliver him the nomination, or allow Ted Cruz to force a contested convention in July.

As in 1964, the race that will culminate in California is also a battle for the ideological soul of the Republican party. Goldwater’s nomination ensured the GOP would be a vessel for conservatism. On June 7, California’s 5 million registered Republicans will decide whether it remains so.

California’s primary comes at a time when the number of registered Republicans in the state has dwindled. High tax rates and a declining economy have led businessmen and entrepreneurs elsewhere. “Our base is leaving,” says Republican national committeeman Shawn Steel. “We export Republicans.”

The phenomenon has created a gulf between core party activists and the state’s rank-and-file Republicans. The former are by and large traditional conservatives who prefer Cruz to Trump, but there is considerable disagreement as to the ideological makeup of the latter, those whom Steel calls “Walmart moms.”