Wall Street kicked off the week on a dour note, with fresh tariff uncertainty rattling investors, while Tesla shares dropped after CEO Elon Musk announced his political party ambitions. Electric vehicle maker Tesla fell 7% to a near one-month low and was on track for its worst day in over a month.

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.

Tesla shares are sinking roughly 7% in premarket trading after CEO Elon Musk announced he would form a new political party in an escalation of his feud with President Donald Trump.