Tesla shares plunged nearly 7 percent on Monday as investors registered dismay at Mr. Musk’s plans to form a third party and his intensifying feud with President Trump.

Before we assess the eternally unrealized dream of a potent third party in American politics, let’s consider the specific, recently emergent dream of one specific man: Elon Musk.

Musk has become the latest rich egomaniac to fall into this trap.