Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) offers fierce criticism of President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE in his upcoming memoir, decrying the president's attacks on the press and denouncing his rhetoric on refugees.

"The way he speaks about [refugees] is appalling, as though welfare or terrorism were the only purposes they could have in coming to our country," McCain writes in his forthcoming book, "The Restless Wave," according to an excerpt published by NPR.

McCain, a vocal Trump critic, also goes after the president for his insistence that media outlets that cover his administration critically are "fake news," saying that authoritarian governments abroad have echoed that phrase to discredit the press.

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"His reaction to unflattering news stories, calling them 'fake news' whether they're credible or not, is copied by autocrats who want to discredit and control a free press," McCain writes, according to NPR.

McCain, who served as the GOP's presidential nominee in 2008, has been among Trump's most ardent GOP critics in Congress.

While the bulk of McCain's book reportedly deals with other major issues of the past several years, including the Iraq War, the Arizona Republican also offers a biting assessment of Trump.

In one section, McCain admits that he's "not sure what to make of President Trump's convictions."

"He threatened to deliberately kill the spouses and children of terrorists, implying that an atrocity of that magnitude would show the world America's toughness," he added later.

On the campaign trail, Trump railed against the notion of resettling refugees in the U.S., warning that doing so would make the country vulnerable to terrorist plots.

Since taking office, he has significantly curbed the number of refugees admitted to the country and has placed a highly controversial ban on people from several countries — including war-ravaged Syria — from entering the U.S.