Tesla stock lost nearly 7% Monday after Elon Musk announced plans of launching the “America Party.” William Blair downgraded TSLA shares.

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.