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.

In a flurry of late-night posts Monday on his social media platform Truth Social, Trump announced he will purchase “a brand new Tesla” in “a show of confidence and support” for Elon Musk, who is leading his efforts to slash the federal government.