Tesla stock is weaker in premarket trade as investors react to Elon Musk’s announcement that he will form a new U.S. political party.

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.