Before 1994, Republicans had won three consecutive elections for governor. Since then, they’ve lost four of five regularly scheduled contests.

In the meantime, Republicans have lost all six Senate elections. In four of those races, Republican candidates failed to muster 40 percent of the vote.

The surge by Democrats in the nation’s largest state is not difficult to explain. Mr. Wilson won in 1994 by dominating among white voters, who exit polls conducted by the Voter News Service showed represented 78 percent of the electorate. Latinos were 9 percent, blacks 7 percent and Asians and other minorities 6 percent.

By the 2014 election, whites had declined to 59 percent of the vote, according to exit polls conducted by Edison Research. Latinos swelled to 18 percent, Asians and other minorities to 14 percent, and blacks edged up to 8 percent. Those nonwhite constituencies disproportionately backed the incumbent governor, the Democrat Jerry Brown, Ms. Brown’s brother.

“Republican leaders and candidates were just too slow to understand what their demographic destiny was,” said Mr. Stutzman.

Lately, California Republicans have worked to lure back Asian-American voters, who at one time were attracted to the Republicans’ economic and national security policies. And they have seen an increasing number of younger Latinos register with no party preference, rather than as Democrats.

Enter Mr. Trump, with his call for a temporary ban on entry of Muslims into the United States, his description of some Mexican immigrants as “criminals” and “rapists,” and his remarks about the “Mexican heritage” of a federal judge who was born in Indiana.