President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE lowered the White House flags to half-staff on Saturday following the news of Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE's (R-Ariz.) death.

McCain, a giant of the Senate, died Saturday afternoon at age 81 following a battle with an aggressive form of brain cancer.

Trump paid tribute to McCain on Twitter, shortly after McCain's office announced the senator's death, writing, "My deepest sympathies and respect go out to the family of Senator John McCain. Our hearts and prayers are with you!"

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Arizona Gov. Doug Ducey (R) tweeted that he ordered all flags to be lowered to half-staff following news of McCain's death.

"May God rest his soul and look over his entire family. Our state and our nation mourn together," Ducey wrote on Twitter.

McCain survived years as a prisoner of war in Vietnam before rising to become a maverick of the Republican Party and a leading actor on the political stage. He led a storied career spanning decades in the Senate.

Despite his absence from the Senate this year, McCain was ever-present in Trump's time in the White House.

McCain frequently criticized Trump, from his trade policies to his criticisms of the media. Trump, too, often sparred with McCain.

Then-candidate Trump famously mocked McCain for being captured during the Vietnam War, saying “he’s not a war hero” and that he “[likes] people who weren’t captured.”

The Washington Post reported earlier Saturday that Trump said he does not regret the comment.

McCain reportedly did not want Trump to attend his funeral.

— Updated Aug. 26, 12:02 a.m.