During a feud with President Trump, Tesla Chief Executive Elon Musk announced plans to start his own political party. Shares for his company fell more than 6% Monday.

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.