Mr. Musk’s involvement in politics and his financial support for the president’s campaign were once seen by investors as a benefit to Tesla, fueling a steep rise in company shares after the election last year. That association has now turned into a liability. Tesla shares are down 40 percent from their peak in December.

Musk said he wants to form the America Party. A third party has never won the presidency, but they have influenced elections, experts say — mostly as spoilers.

Elon Musk's net worth has taken a $15.3 billion hit after the Tesla CEO announced he was to form a new political party. The news sent ripples through financial markets and spooked investors concerned about Musk's political ambitions and possible implications for Tesla's brand and business operations.