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BURLINGAME, Calif. — Former Gov. Pete Wilson of California used a surprise appearance before the state’s Republican convention on Saturday to endorse Senator Ted Cruz, warning that the nomination of Donald J. Trump could lead to political ruin for the party’s candidates this November.

“My friends, we cannot afford a Republican nominee that brings us down-ticket decimation,” said Mr. Wilson, pointing to Mr. Trump’s weakness among women voters.

He also used his remarks to argue that Mr. Trump could not be trusted with Supreme Court appointments, an issue of great importance to the highly engaged conservative activists gathered in a hotel ballroom here near San Francisco.

“Heaven knows what justices Donald Trump would pick,” said Mr. Wilson, adding: “We can’t afford a wild card when it comes to the president who will be making critically important Supreme Court appointments.”

The Republicans in attendance, many of them supporters of Mr. Cruz, responded warmly. But Mr. Wilson, 82, a former senator as well, has been out of office since 1999 and was raising issues that today’s Republican primary voters have largely dismissed as Mr. Trump has piled up an overwhelming delegate advantage. Traditional Republican leaders and activists are deeply fearful of Mr. Trump’s candidacy, but their arguments against him have largely fallen on deaf ears.

There was also something rich about Mr. Wilson’s blunt words of caution: Many Republicans in attendance at the state party’s convention fear that Mr. Trump’s stridency on the immigration issue could do the national party what they believe Mr. Wilson’s approach to the issue did to California Republicans. His support as governor for Proposition 187 — the successful, but ultimately overturned, 1994 ballot initiative to deny health care, public education and other social services to illegal immigrants — is widely seen as having damaged the party’s ability to win statewide elections here. (Since the ballot initiative, no Republican presidential candidate has won California, and the state has elected only one Republican governor or senator, Arnold Schwarzenegger, a centrist.)

Mr. Cruz used his remarks to promise a hard line on illegal immigration but did not dwell on the topic, instead faulting Mr. Trump on taxes, health care, guns and foreign policy.

He did, though, incorporate a locally themed attack in his speech. He noted that Mr. Trump had given $12,000 to some of the present and probable future leaders of the California Democratic Party: Gov. Jerry Brown, Lt. Gov. Gavin Newsom and Attorney General Kamala Harris, who are generally reviled by this state’s beleaguered Republican Party.

“Y’all are experiencing firsthand the consequences of those misguided choices,” said Mr. Cruz, drawing boos with his mention of the California Democrats.