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.

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 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.