President Trump inserted his considerable influence into the race for governor of New Jersey on Monday, announcing his endorsement of Jack Ciattarelli, a Republican who came within three points of defeating the state’s Democratic governor in 2021.

“Jack Ciattarelli is a terrific America First Candidate running to be the next Governor of a State that I love, NEW JERSEY!” Mr. Trump wrote on Truth Social.

“He is strongly supported by the most Highly Respected Leaders in New Jersey and, as your next Governor, Jack Ciattarelli will work closely with me and the Trump Administration.”

The public support from Mr. Trump is likely to solidify Mr. Ciattarelli’s odds in the June 10 Republican primary. He is competing against two prominent opponents — Bill Spadea, a conservative talk show host who had been similarly vying for the president’s endorsement, and State Senator Jon Bramnick, a moderate who has been an outspoken critic of the Trump administration.

Mr. Ciattarelli, 63, a former member of the State Assembly, said he was “truly humbled and honored to receive President Trump’s very strong endorsement.”

“It’s time to unite our party, win big in November and make New Jersey affordable and safe again,” Mr. Ciattarelli wrote on social media.