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.

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.

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.