The hostility goes back to the 2016 campaign, when Trump disparaged the Arizona Senator, by declaring McCain was "not a war hero" for serving in the Vietnam War.
"He's not a war hero," Trump said of McCain in 2015.
"He was a war hero because he was captured. I like people who weren't captured."
During a Republican Veterans for Biden-Harris panel, Cindy McCain said that in the months leading up to Trump's election: "I listened to him denigrate my husband, and then denigrate my husband again while [John] was on his death bed."
Biden has been a longtime friend of the McCain family, bonding not only as the parents of children who have served in the military, but also over glioblastoma – the aggressive brain cancer that killed Biden's son Beau three years before McCain succumbed to the same disease.
"Now more than ever," Cindy McCain said in the Biden campaign's first TV ad, "we need a president who puts service before self."
McCain said Biden would "always fight for the American people, just like John did".
Hours after Trump addressed the yesterday to claim the election had been stolen from him, John McCain's 2008 concession speech went viral.
McCain, who lost his presidential run against former President Barack Obama 12 years ago, began his speech by congratulating Obama on his presidency and asking his supporters to respect the will of the people.
John McCain died in August, 2018.
Barack Obama delivered a stirring eulogy in his honour.