OTTAWA—Canada’s powerhouse ambassador to Washington Kirsten Hillman is stepping down in a surprise resignation as critical trade negotiations have stalled and the U.S. is toying with letting the continental free trade pact expire.

In a public statement posted late Tuesday, Hillman did not reveal the reason she is leaving now but she previously had been expected to stay on for a full year, through next year’s review of the Canada-US-Mexico agreement.

Hillman has a famous Rolodex of contacts in Washington and state legislatures, and contacts in both Republican and Democratic political circles.

She steered Canada’s response to the Trump administration and its aggressively protectionist second term.

One of the lead trade negotiators of the Comprehensive Trans-Pacific trade partnership that

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