There are plenty of good reasons to criticize how Justin Trudeau's Liberals have handled Canada's foreign affairs and Andrew Scheer is perfectly within his rights to do so.

But in recent days, the Conservative leader and his party crossed a line into unacceptable territory, not only by unfairly faulting Trudeau for the ongoing detention of two Canadian citizens in China, but by exploiting those prisoners to serve partisan needs.

The Conservatives should step back over that line ASAP. In the nationally important cause of freeing Michael Spavor and Michael Kovrig, Canadian politicians should work together, not against each other.

Scheer must realize the real target of his censure should be China's authoritarian regime which, since last December, has held Spavor and Kovrig in appalling conditions.

Their incarceration is widely seen as naked Chinese retaliation for the decision by Canadian authorities to detain Chinese businessperson Meng Wanzhou in response to a request by the Americans. They want to extradite the Huawei executive to the U.S. to face charges of violating sanctions against Iran.

Scheer's desire to free the Canadians - who are being held on dubious allegations they violated Chinese security - is laudable. His tactics are deplorable.

In a fundraising video slamming Trudeau, the Conservatives named both Spavor and Kovrig and used their photographs. Last week, the Tories removed their names and images from the video after the family of one of the men understandably complained.

But that doesn't excuse the Conservatives for exercising terrible judgment in the first place. It was insensitive of the party to use the two men in this way. It exploited them and the suffering they must be enduring, using them as poster children to rally the Conservative base and raise a few bucks for the coming federal election campaign.

This gaffe will remind many Canadians that the Conservatives pulled an ad last year that showed a black man crossing the border from the U.S. into Canada as part of the party's criticism of how the Liberals have handled an influx of irregular migrants.

This time, the problem goes beyond a tasteless video. Scheer is blaming Trudeau for the plight of Spavor and Kovrig, claiming the PM has "failed to negotiate the release of two Canadians arbitrarily imprisoned in China."

The problem isn't Trudeau. It's the Chinese government which is not bound by any notion of the rule of law and which has also lashed out against this country by suspending all imports of Canadian canola, pork and beef.

There's no proof getting tougher with China, as Scheer advocates, would win the two prisoners' release. As the world's most populous nation and its second biggest economy, China is not easily moved, particularly by threats.

More to the point, China is holding the two Canadians in jail precisely as a club to batter the Canadian government into releasing Meng. In trying to bully the Canadian government, the Chinese are searching out weaknesses.

Dividing and conquering appeals to them. And in pointing a wagging finger at Trudeau, Scheer is unwittingly playing China's game.

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To be sure, the Conservatives are not alone in running offensive ads. Some of the Liberal ads targeting Scheer, for instance, have come close to crossing the line. The current state of political advertising in Canada is dreadful. It encourages polarization more than it fosters democratic participation.

But on this day, it's Scheer who needs the reality check. It's important for his future that Canadians know him better. What they've come to know about him in this case, however, will do him no favours.

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