Chrystia Freeland's Aunt Natalka used to be a diplomatic asset.

Last summer, the Foreign Affairs Minister told The Globe and Mail that she had built such a close relationship with U.S. Commerce Secretary Wilbur Ross that he had become acquainted with her aunt via his frequent phone calls to the Freeland residence.

It was evidence of the Trudeau government's charm offensive south of the border, where cabinet ministers and the PM himself seemed to be spending every free minute pressing the Canadian case for continental free trade with their counterparts in the Trump administration, as well as with state governors and other non-federal officials.

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At the time, Mr. Ross was one of Mr. Trump's key point people on trade and seemed to have major clout in the White House. Ms. Freeland's courtship looked like a coup.

Not any more. The U.S. news site Axios reported last week that Mr. Ross has lost much of his influence ever since Mr. Trump suddenly turned against him and began trashing him to colleagues in humiliating terms.

So much for Canada's courtship of Mr. Ross.

This is what happens to anyone trying to sway Mr. Trump, or to forge a deal with him. It was the fate of Democratic Party leaders who repeatedly thought they were close to reaching an agreement with the President to reform immigration and prevent a government shutdown last week, only to have him back out. "Negotiating with President Trump is like negotiating with Jell-O," said Senate Minority Leader Chuck Schumer on Saturday. "It's next to impossible."

You don't have to tell the Trudeau government. Heck, you don't have to tell anyone at this point. That Mr. Trump is erratic has become a truism. But as NAFTA talks resume in Montreal this week, it's worth asking why he makes such a slippery interlocutor, and what Canada can do about it.

Part of the answer begins with Mr. Ross. He is among a growing troupe of Trump administration officials who have been ejected from the President's inner circle with terrifying force and speed, often while being bashed in the press, or on Mr. Trump's Twitter account, on the way out.

This has led Canadian officials down all manner of blind alleys. Think of the apparent rapport between Mr. Trudeau's principal secretary, Gerald Butts, and erstwhile Trump consigliere Steve Bannon – exiled from the White House in August and more recently torched by the President for some damaging quotes he gave to the author Michael Wolff. The once-touted cross-border relationship between top strategists has lost all value.

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The trouble is, there's no way to predict who will run afoul of Mr. Trump. He is touchy and moody in the extreme, prone to bouts of seemingly random distemper, and easily offended. He is also utterly treacherous, apparently loyal to no one except perhaps for his daughter Ivanka.

That latter fact suggests that one Canadian strategy could be fruitful: getting in with the family. Ivanka Trump and her husband Jared Kushner are close advisers to the President, and hard to fire. Prime Minister Justin Trudeau has worked hard to woo Ivanka, taking her to a Broadway show and bonding over their shared feminism.

But this points to the deeper difficulty of convincing Mr. Trump that NAFTA is good for the U.S., or convincing him of anything, really: He is so prone to having his mind changed that any Friend of Canada in the Oval Office, be it Ivanka or someone else, is liable to have her work undone by the next person to brief him, or by a talking head on Fox News.

Many people who have dealt with Mr. Trump report that his position on an issue tends to align with that of the last person he spoke to. Since he reportedly likes to watch TV and make phone calls in bed, where he can't be managed by his chief of staff, it's impossible to know who will have his attention during eleventh-hour NAFTA negotiations, except that it won't be someone who resides in Ottawa.

For that reason, the Trudeau government will have to work around the President from here on in. Its ministers and officials should continue to seek allies in Congress, in statehouses and in municipal governments, where people understand the economic devastation that killing NAFTA would bring, and avoid the denizens of a White House who can't even be sure of where they will be working in six months.

Bluntly put, NAFTA is too important to Canada to perch on a plate of Jell-O.