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.) wrote in his upcoming memoir that he does not regret alerting the FBI about the so-called Steele dossier, which details 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’s alleged ties to Russia.

McCain acknowledged in January 2017 that he delivered a dossier of “sensitive information” to then-FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE.

In his upcoming book, "The Restless Wave: Good Times, Just Causes, Great Fights, and Other Appreciations," McCain wrote that he "did what duty demanded I do" in passing on the documents.

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“I discharged that obligation, and I would do it again. Anyone who doesn’t like it can go to hell,” McCain said, according to excerpts of the book published by The Guardian.

He added that any risk that the president might be compromised by Russians "had to be investigated."

The Steele dossier has been at the center of investigations into Russian meddling in the 2016 election conducted by congressional committees and special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE. Trump has repeatedly called the latter's probe a "witch hunt" and a "hoax."

The document has been a source of controversy, as it was funded partly by the Democratic National Committee and Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE’s presidential campaign.

It started out as an opposition research document funded by the conservative website The Washington Free Beacon.

The dossier contains claims about Trump's ties to Russia, including some salacious allegations about the president. Some of the claims have not been verified.

Comey, who was fired in May 2017, has said that Trump was fixated on disproving some of the more scandalous claims in the dossier.