Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.) defended 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 as "one of the most consequential senators" in history amid renewed attacks from 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 against his late friend and GOP colleague.

"As to @SenJohnMcCain and his devotion to his country: He stepped forward to risk his life for his country, served honorably under difficult circumstances, and was one of the most consequential senators in the history of the body," Graham tweeted on Sunday.

"Nothing about his service will ever be changed or diminished."

Nothing about his service will ever be changed or diminished. (2/2) — Lindsey Graham (@LindseyGrahamSC) March 17, 2019

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Graham, who is an ally of the president and was one of McCain's closest friends in the Senate, spoke out hours after Trump issued his second tweet in as many days targeting McCain.

The president disparaged McCain over reports that one of the then-senator's associates had shared a dossier of allegations about Trump's ties to Russia with the media.

"So it was indeed (just proven in court papers) 'last in his class' (Annapolis) John McCain that sent the Fake Dossier to the FBI and Media hoping to have it printed BEFORE the Election," Trump tweeted on Sunday. "He & the Dems, working together, failed (as usual). Even the Fake News refused this garbage!"

On Saturday, the president quoted former independent counsel Ken Starr, who called it a "very dark stain" against McCain that he had helped spread the dossier.

"He had far worse 'stains' than this, including thumbs down on repeal and replace after years of campaigning to repeal and replace!" the president tweeted, referring to the Arizona lawmaker's vote against the GOP Senate majority during the Republican attempt to repeal ObamaCare in 2017.

Trump and McCain had a fraught relationship, with the Senate icon remaining one of the president's staunchest Republican critics even after his cancer diagnosis.

McCain died last August, but the president has occasionally brought up his vote against the GOP effort to repeal ObamaCare in 2017 since then.