Barack Obama made an unusual endorsement of a foreign leader on Wednesday, tweeting that "the world needs" the "progressive leadership" of Canadian Prime Minister Justin Trudeau.

Why it matters: Trudeau is in a tight race ahead of Monday's election. His rock-star image has been battered by an ethics scandal and newly surfaced images of him in blackface. Obama is lending a helping hand to a leader he said he was "proud to work with" while in office.

Go deeper: Justin Trudeau's political star fades ahead of Canada's election