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.

President Donald Trump said he would be buying a “brand new” Tesla to support his senior adviser Elon Musk, as the stock market shows shares of the electric car company tumbling.