Federal Conservatives are mounting an aggressive cross-border campaign against Prime Minister Justin Trudeau for his government's $10.5-million payment to Omar Khadr, appearing in U.S. media outlets and launching a website criticizing the Liberal government's decision to compensate and apologize to the former child soldier and Guantanamo detainee.

This week, Conservative MP and former journalist Peter Kent drew significant U.S. attention to the case after he wrote an opinion piece about the payment in the Wall Street Journal, calling it a "cynical subversion of Canadian principles," while Conservative MP Michelle Rempel appeared on Fox News with host Tucker Carlson to lambaste the government.

"Most Canadians are absolutely outraged about this," Ms. Rempel said during her in-studio visit. When Mr. Carlson asked if the payment was "a way of giving the finger to the United States," Ms. Rempel replied, "I think that this should have played out in a court of law."

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Ontario Conservative MP Cheryl Gallant also targeted Canadian journalists in a faux newscast-style Facebook video, accusing the "elite-stream media" of "fake news" stories about Mr. Khadr to keep the Liberals in power. The video has since been taken down, but was saved by Press Progress, the media project from the left-leaning Broadbent Institute.

The federal Conservative Party recently launched a website, called "Khadr Questions," with suggested social-media posts questioning why and when Mr. Trudeau's government made its decision to compensate "a convicted and self-confessed terrorist."

"Justin Trudeau has made Khadr one of the wealthiest men in Canada," it says.

The Liberals quickly shot back at the opposition. Stephen Fuhr, the Liberal MP for Kelowna-Lake Country in British Columbia, called the Tory website an example of the "unacceptable politics of fear and division that has become a hallmark of [leader] Andrew Scheer," in a statement provided by the Prime Minister's Office.

One of Mr. Trudeau's top advisers, Gerald Butts, weighed in on Twitter, suggesting Mr. Scheer's Conservatives have abandoned the bipartisanship that defined interim leader Rona Ambrose's tenure. "The Canada-U.S. relationship should be above domestic politics. For a brief moment, between permanent CPC leaders, it was," he wrote in one tweet.

In reference to Mr. Kent's article, Mr. Butts tweeted: "A long CPC tradition of airing Canadian disputes in the Wall Street Journal. It's where Stephen Harper advocated for joining the Iraq War." He also said Mr. Khadr "beat the federal government three times at the Supreme Court" because he "got tortured in Guantanamo."

Mr. Trudeau has publicly defended the government's apology and settlement, saying Mr. Khadr's rights had to be defended.

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"The Charter of Rights and Freedoms protects all Canadians, every one of us, even when it is uncomfortable," Mr. Trudeau said at the end of this month's Group of 20 Summit in Hamburg, Germany. "This is not about the detail of the merits of the Khadr case. When the government violates any Canadian's Charter rights, we all end up paying for it."

The Toronto-born Mr. Khadr was captured in Afghanistan at the age of 15 in 2002, following a shootout with U.S. troops where he was badly wounded. He was accused of throwing a grenade that killed U.S. army medic Christopher Speer in the firefight and was sent to the U.S. detention facility in Guantanamo Bay, Cuba.

Mr. Khadr, now 30, spent more than 10 years in U.S. and Canadian custody, much of that time in Guantanamo. Once the youngest detainee there, he was transferred to Canada in 2012 after accepting a plea deal.

He now argues the acts he is accused of committing as a 15-year-old in Afghanistan were not war crimes at the time and says he only pleaded guilty to throwing the grenade that killed Mr. Speer as a way out of American captivity.

The Supreme Court of Canada ruled in 2010 that the actions of federal officials who participated in U.S. interrogations of Mr. Khadr had offended "the most basic Canadian standards about the treatment of detained youth suspects." At the U.S. military prison, Mr. Khadr was subjected to physical pain, isolation, sleep deprivation, shackling in stressful positions and threatened with rape.

Mr. Khadr's lawyers had filed a $20-million lawsuit against the federal government. Mr. Trudeau recently said he understands why Canadians are angry about the payout, but insisted a court case would have ended up costing taxpayers tens of millions of dollars more. "I share those concerns about the money. In fact, that's why we settled," Mr. Trudeau told reporters last week.

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The payment stymied efforts by Mr. Speer's widow, Tabitha Speer, and Layne Morris, a fellow Delta Force soldier who was blinded in one eye during the 2002 firefight, to stop the government from settling with Mr. Khadr.

Ms. Speer and Mr. Morris won a 2015 default judgment in Utah against Mr. Khadr for $134-million (U.S.) in damages for his alleged actions in Afghanistan, and want courts in Canada to recognize and enforce that judgment. Mr. Khadr was in prison and did not defend himself.

Conservative Party spokesman Cory Hann said the Khadr website was created "due to the overwhelming amount of questions Canadians have surrounding the $10-million taxpayer-funded payout to an admitted and convicted terrorist." The Liberal government has faced a public backlash against the apology and payment to Mr. Khadr, with a public-opinion survey showing 71 per cent of Canadians opposed the deal.

Mr. Hann denied, however, that the personal data collected on the site will be used for fundraising purposes.

"This is not a fundraising website and no fundraising will be done off it. It's meant to keep people up to date on the story and allow them to share the questions they have that the Liberals refuse to answer," he wrote.

An online fundraiser set up by right-wing Rebel Media, which aims to raise $1-million for Mr. Speer's two teenage children, had raised almost $200,000 by midday Tuesday.

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Matthew Dubé, the NDP's public safety critic, blamed both the former Conservative and Liberal governments for causing the controversy in the first place.

"New Democrats have always maintained that Omar Khadr's case should be handled by Canadian authorities in accordance with Canadian law, free from political interference. It's very troubling to see Conservatives continue to play politics by fundraising off the case," he said in a statement.

"If past Liberal and Conservative governments had done their jobs, there would be no need for this compensation."

With a report from the Canadian Press