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.

Shares in Tesla tanked by as much as 7.6% in premarket trading Monday after its CEO Elon Musk said he is forming a new American political party, provoking an irate response from US President Donald Trump.