Sen. Chris Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) on Sunday called on 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 to apologize for tweets attacking the late 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).

"I've long thought that his personal and direct attacks on Sen. McCain was one of the most detestable things about President Trump's conduct as a candidate," Coons said on ABC’s “This Week.”

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Trump earlier Sunday criticized McCain for a second consecutive day. The criticism follows reports that an associate of the late Arizona Republican 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. "He & the Dems, working together, failed (as usual). Even the Fake News refused this garbage!"

The president in a tweet on Saturday quoted former independent counsel Ken Starr, who called it a "very dark stain" that McCain had helped spread the dossier.

“Sen. McCain conveyed that report out of a sense of duty, and he is someone who lived his entire life with a sense of duty to our country,” Coons said on Sunday. “And I frankly think the president’s continued attacks on now late Sen. John McCain is something that’s regrettable and for which he should apologize.”

Trump and McCain had a fraught relationship during the senator's life.

The president during his campaign in 2015 went after McCain by suggesting he was not a war hero because he was captured while serving in Vietnam. He has also repeatedly brought up the late-senator's "no" vote on a 2017 bill repealing the Affordable Care Act, and later delayed officially ordering flags lowered to half-staff following McCain's death.

McCain's daughter, Meghan McCain, ripped Trump after his tweet on Saturday.

"No one will ever love you the way they loved my father.... I wish I had been given more Saturday’s with him. Maybe spend yours with your family instead of on twitter obsessing over mine?" she tweeted.