Mr. Trump’s endorsement tweet was even pasted to the bottom of news releases sent by the campaign.

Mr. Cox has not only rejected the negative attacks on the White House by California Democrats, but has also sought to flip those critiques back onto liberals by emphasizing high taxes and high rates of poverty and homelessness in the state. “It wasn’t Donald Trump that made California the highest-taxed state in the country; it was Gavin Newsom and the Democrats,” he said.

Mr. Newsom, for his part, has already shown his desire to frame the contest as a referendum on Mr. Trump’s agenda, and has painted Mr. Cox as a surrogate for the president.

“We’re engaged in an epic battle, and it looks like voters will have a real choice this November — between a governor who is going to stand up against Donald Trump and a foot soldier in his war on California,” Mr. Newsom told supporters in San Francisco.

Mr. Cox faces long odds in California, with or without Mr. Trump’s support.

Mr. Newsom “can run as hard left as he wants and as long as he attacks Trump two or three times a day, he’ll be just fine,” said Dan Schnur, who was an adviser to the former Republican governor Pete Wilson. “The only thing better than winning big for Newsom would be to goad Trump into the race.”

More election coverage from The Times:

• Read our story about the sharp ideological differences between Mr. Newsom and Mr. Cox, with more analysis about Mr. Trump’s presence in the race.