PHOENIX (AP) — Democrat Joe Biden said Tuesday that Cindy McCain plans to endorse him for president, a stunning rebuke of President Donald Trump by the widow of the GOP's 2008 nominee.

Trump has had a fraught relationship with members of John McCain's family since he disparaged the Arizona senator during his 2016 campaign. But the McCains have stopped short of endorsing Trump's rivals.

Cindy McCain's backing could help Biden appeal to Republicans disaffected with the GOP president and give the former vice president a boost in Arizona, a crucial swing state that McCain represented in Congress for 35 years. He's remained a revered figure since his 2018 death from complications of a brain tumor, particularly with the independent voters whom Biden is courting.

Biden told donors on Tuesday evening that McCain’s endorsement was coming “because of what (Trump) talks about how my son and John and others who are heroes, who served their country. You know, he said they’re ‘losers, suckers.’”

Biden was referring to comments Trump reportedly made mocking the American war dead. Trump has denied making the remarks, first reported through anonymous sources by The Atlantic, but many of the comments were later confirmed by The Associated Press.

Cindy McCain had not initially been expected to offer an explicit endorsement of Biden, but she had already gone to bat for his presidential run. She lent her voice to a video that aired during the Democratic National Convention and was focused on Biden’s close friendship with her late husband.

John McCain was assigned to be a military aide for Biden, then a senator, during an overseas trip, and their families formed an enduring friendship. They later shared a grim bond over glioblastoma, an aggressive cancer that killed Biden's son Beau three years before McCain succumbed to the same disease.

John McCain said in 2016 that he couldn’t support Trump or Democrat Hillary Clinton in 2016, citing Trump's demeaning comments about women.

“It’s not pleasant for me to renounce the nominee of my party,” McCain said during a debate as he sought his sixth term in the Senate. “He won the nomination fair and square.”

A Navy pilot, John McCain was shot down over North Vietnam in 1967. He was captured, beaten and held prisoner for more than five years, refusing to be released ahead of other American servicemembers.

During his 2016 campaign, Trump said of McCain, “He’s not a war hero. He was a war hero because he was captured. I like people who weren’t captured.” McCain later angered Trump with his dramatic thumbs-down vote against repealing Obama’s health care law.

The McCains’ daughter Meghan McCain has been outspoken about the pain she feels when the president disparages her father. Biden consoled Meghan McCain on an appearance on “The View” after her father was diagnosed with the cancer that eventually took his life. She has said Biden often reaches out to her to offer support, after losing his own son Beau to the same cancer in 2015.

Trump wasn’t invited to John McCain’s funeral.