WASHINGTON — The difficult diplomacy Canada is attempting with the incoming Trump administration took on new pressure as Prime Minister Justin Trudeau announced he will resign his post as soon as a new Liberal leader is chosen. Trudeau’s decision… brings instability to Ottawa just as Canada is trying to tamp down Donald Trump’s threat of devastating tariffs. …”I don’t think a leadership change could come at a worse moment in our bilateral relationship,” said Fen Osler Hampson, at Carleton University and co-chair of the Expert Group on Canada-U.S. Relations. …President-elect Trump responded to Trudeau’s announcement saying the prime minister is resigning because he knows the U.S. will not put up with trade deficits with Canada. …Observers also say Trump only negotiates with people who have power. Hampson said Trump will not “waste his time with Ottawa.” That means, Hampson said, premiers are in a position where they will have to carry more of the bilateral burden.