Shares in Tesla plunged 6.79% Monday after its CEO Elon Musk said he is forming a new American political party, provoking an irate response from President Donald Trump. Shares in Tesla (TSLA) had dropped as much as 7.

Mr. Musk’s involvement in politics and his financial support for the president’s campaign was 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.

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.