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.

Tesla shares sank Monday after CEO Elon Musk announced he would form a new political party in an escalation of his feud with President Donald Trump.