Elon Musk, the billionaire head of Tesla and SpaceX, strode onto the stage to cheers at Donald J. Trump’s rally on Saturday night, lifted his arms above his head and jumped into the air — twice — exposing his navel as his shirt rode up.

Wearing an “Occupy Mars” shirt underneath a sport coat, he nodded to his black “Make America Great Again” baseball cap.

“As you can see, I’m not just MAGA, I’m dark MAGA,” he said.

Mr. Musk publicly endorsed Mr. Trump in the minutes after a gunman tried to kill the former president on July 13 in Butler, Pa., in a post on X, the social media platform he owns. So when Mr. Trump returned Saturday to hold a rally at the same venue where he was attacked, he brought Mr. Musk along.

In addition to bouncing up and down onstage, he urged the crowd to “Fight! Fight! Fight!” — an echo of the words Mr. Trump had uttered after the attack.

The crowd seemed to know Mr. Musk, who drew cheers as he was introduced by the former president. Mr. Trump lavished praise on Mr. Musk, the world’s richest man, for building an American car company and saving “free speech” with X.

"President Trump must win to preserve the Constitution,” Mr. Musk said, after bounding to the mic with his hands in the air. “He must win to preserve democracy in America.”

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