It was too much to hope that the uncharacteristic spirit of cross-partisan co-operation would last.

After Donald Trump's election victory last November, the federal Liberals and Conservatives decided that just this once – in the face of an unusual threat to Canada's relationship with its biggest trading partner – they would act like friends.

Rona Ambrose, the interim Official Opposition leader, joined Ottawa's charm offensive in Washington, aimed at educating or reminding Americans the relationship's mutual benefits; true to her word, the Tories resisted attacking the government in ways that could negatively affect looming NAFTA negotiations. Chrystia Freeland, the Foreign Affairs Minister, went out of her way to praise Ms. Ambrose for being so magnanimous. Members of both parties conceded that being united in common purpose was an interesting new experience.

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But these are partisans we're talking about. Once the Conservatives had in place someone to lead them into the next election, there would be less inclination to be charitable than under a placeholder. Whether the opportunity to score points proved too much to pass up, or because of irreconcilable differences in worldview, anything related to Canada-U.S. relations wasn't going to stay walled off from open debate in perpetuity.

Still, it has been remarkable to witness how rapidly the goodwill has eroded, as tensions over the Omar Khadr settlement have spilled over the border – or, really, been aggressively brought to the attention of the same Americans to whom Canadian politicians were singing from the same songbook.

For the first couple of weeks that controversy raged in Canada over the payout to end Mr. Khadr's lawsuit against the federal government for failing to uphold his rights as a citizen while he was detained at Guantanamo Bay, there wasn't much discernible tie-in to current U.S. relations. Stephen Harper personally called Tabitha Speer, the widow of the U.S. Army combat medic allegedly killed by Mr. Khadr in Afghanistan, and Layne Morris, the U.S. soldier wounded in the same firefight, to apologize for Mr. Khadr receiving a reported $10.5-million; an online fundraising campaign affiliated with the right-wing media outlet The Rebel was launched to help raise money for their families. But the federal opposition's criticism of the deal was aimed at Canadians, and American media took little notice.

That changed on Monday, when the Wall Street Journal published an op-ed by Conservative MP Peter Kent under the headline "A terrorist's big payday, courtesy of Trudeau," calling the settlement "an affront … to our U.S. allies." Quickly it became the day's hottest story on Fox News, which the Tories encouraged by having another caucus member, Michelle Rempel, go on Tucker Carlson's evening show.

There are plenty of plausible explanations for why the Conservatives went this route. Their preferred one is that they genuinely want Americans to know they think they've been wronged. On Tuesday, some Tories suggested it's also useful to put a dent in fawning foreign coverage of Mr. Trudeau. They probably figure U.S. attention will help keep the issue, which resonates hugely with their supporters, top of mind in Canada. They may be getting caught up in the thrill of reaching audiences exponentially larger than back home.

Whatever their motives, they broke no established rule. Opinions vary about how much partisanship should normally be set aside when representing Canada abroad. Mr. Trudeau uses the international stage to advance his brand; it's not crazy for Tories to think they should do likewise.

But in how they've chosen to break the cross-border common front, the Conservatives have taken a couple of gambles.

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Domestically, they could be overplaying their hand. The Tories clearly believe that, as polls have suggested, a majority of Canadians (including some Liberals) agree with their opposition to the Khadr settlement. If so, there may be more risk than reward in aligning on the issue with U.S. media outlets that can be off-putting to all but hard-core conservatives. It could cause some of those same Canadians to reconsider which side of this debate they want to be on, and create or reinforce negative associations in how they view Andrew Scheer's party.

And in terms of U.S. politics – well, the current President has been known to make pronouncements based on whatever he happens to see while devotedly watching Fox News. It is not far-fetched to imagine him souring on Mr. Trudeau's government based on what aired on Monday evening, just as he might have wanted a diversion from the collapse of Republican health-care legislation, and a few weeks from the start of talks to renegotiate NAFTA.

There are no signs yet that Mr. Trump is taken with the Khadr saga. But even if U.S. media moves on soon, and the issue blows over there without consequence, something seems to have changed in the face Canada puts forward.

Late on Monday night, Conservative foreign affairs critic Tony Clement responded to the release of worrying but vaguely worded U.S. objectives for the NAFTA negotiations by declaring, on Twitter, that "PM Trudeau's much-vaunted U.S. strategy is already in tatters."

Perhaps Mr. Clement was overexcited because of his party's big day south of the border. But it wasn't the sort of thing you'd have heard from the Conservatives on this subject until very recently. Despite Canada-U.S. relations being the most important matter facing Mr. Trudeau's government, or perhaps because of that, the opposition seems to have gone back to opposing.