Canadian Prime Minister Justin Trudeau is facing the biggest test of his political career following the resignation of Finance Minister Chrystia Freeland, long one of his most powerful and loyal ministers.

Canadian Finance Minister Chrystia Freeland quit on Monday after clashing with Prime Minister Justin Trudeau on issues including how to handle possible U.S. tariffs, dealing an unexpected blow to an already unpopular government.

Chrystia Freeland, the deputy prime minister and finance minister, had been helping lead Canada’s response to the incoming Trump administration.