Mr. Musk is fundamentally a creature of Silicon Valley, a scene that initiated an entire revolution in power dressing. The young masters of that universe dressed down to show that they were members of a new elite — one that refused to dress like the grown-ups they were about to topple. It was the defeat of the suit by the T-shirt.

Squinting in the sun with Elon Musk, Trump stood next to five Tesla vehicles, holding a piece of paper with handwritten notes about their features and costs. Trump said he would purchase a car himself at full price.