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Cindy McCain, widow of Sen. John McCain, lent her voice to a video during Tuesday's second night of the Democratic Convention that highlighted her late husband's close friendship with presidential nominee Joe Biden.

"It was a friendship that shouldn't have worked," she says in the video, describing the relationship of her late husband, an Arizona Republican, and Biden, a Delaware Democrat.

She explains that her husband and Biden got to know each other in the 1970s, when McCain -- newly home from service in the Navy -- was assigned to be a military aide for Biden and they logged thousands of miles together on overseas trips.

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Cindy McCain waves to the crowd after being acknowledged by Arizona Republican Gov. Doug Ducey during his State of the State address.

Cindy McCain waves to the crowd after being acknowledged by Arizona Republican Gov. Doug Ducey during his State of the State address. (Associated Press)

John McCain is heard in the video claiming his job was to carry Biden's bags.

"The son-of-a-gun never carried my bags," Biden answers back. "He was supposed to carry my bags, damn it, but he never carried my bags."

Cindy McCain's participation in the video was among attempts by Democrats to show Biden has attracted the support from some longtime Republicans as he looks to deny President Trump a second term in November's election.

But while McCain speaks glowingly of Biden in the video she falls short of offering an explicit endorsement of his candidacy.

John McCain was the 2008 Republican presidential nominee against Barack Obama, who won the election with Biden as his running mate.

Cindy McCain is the latest Republican to join the Democrats' convention. A number of notable GOP former elected officials — including former Ohio Gov. John Kasich and former New Jersey Gov. Christine Todd Whitman — endorsed the presumptive Democratic presidential nominee Monday night.

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Both Cindy McCain and her daughter Meghan have been outspoken critics of President Trump, and the family is longtime friends with the Bidens. Trump targeted John McCain personally in 2015, saying the former prisoner of war wasn't a hero "because he was captured. I like people who weren't captured."

McCain later angered Trump with his dramatic thumbs-down vote against repealing President Obama's health care law. Before McCain died of cancer in August 2018, reports suggested he made an explicit request that Trump not be invited to his funeral.

Sen. John McCain, R-Ariz., receives the Liberty Medal from Chair of the National Constitution Center's Board of Trustees, former Vice President Joe Biden, in Philadelphia.

Sen. John McCain, R-Ariz., receives the Liberty Medal from Chair of the National Constitution Center's Board of Trustees, former Vice President Joe Biden, in Philadelphia. (Associated Press)

Biden consoled Meghan McCain during 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.

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Though Meghan suggested in April she'd vote for Biden, Cindy has not made any explicit endorsements. In April of last year, amid rumors that the McCains would wade into the election in support of Biden, Cindy McCain tweeted that Biden was "a wonderful man and a dear friend of the McCain family."

"However," she added at the time, "I have no intention of getting involved in presidential politics."

The video on Biden's friendship with McCain is one of a series of short documentaries created by Oscar Award-winning director Davis Guggenheim, who worked on "An Inconvenient Truth," the 2006 documentary on climate change that featured former Vice President Al Gore.

The Associated Press contributed to this report.