By Eric Bradner and Sarah Mucha, CNN

(CNN) -- Cindy McCain, the widow of longtime Republican Sen. John McCain of Arizona, said Tuesday night on Twitter that she endorses Democratic presidential nominee Joe Biden.

The endorsement follows McCain's appearance in a video about Biden's relationship with her late husband at the Democratic National Convention in August.

McCain offered her endorsement on Twitter after Biden had told donors she was supporting him.

"My husband John lived by a code: country first. We are Republicans, yes, but Americans foremost. There's only one candidate in this race who stands up for our values as a nation, and that is @JoeBiden," she said in a series of tweets.

"Joe and I don't always agree on the issues, and I know he and John certainly had some passionate arguments, but he is a good and honest man. He will lead us with dignity," she said. "He will be a commander in chief that the finest fighting force in the history of the world can depend on, because he knows what it is like to send a child off to fight."

Biden tweeted in reply, "Cindy — I'm deeply honored to have your support and your friendship. This election is bigger than any one political party. It requires all of us to come together as one America to restore the soul of the nation. Together, we'll get it done."

At a virtual fundraiser Tuesday, Biden had told attendees that Cindy McCain's decision to endorse him followed President Donald Trump's comments, as reported by The Atlantic, referring to those killed and injured in wartime as "losers" and "suckers." John McCain was a prisoner of war in Vietnam after his plane was shot down on a bombing mission. The injuries he sustained in Vietnam led to lifelong disabilities, including the inability to raise his arms over his head.

"Maybe I shouldn't say it, but I'm about to go on one of these Zooms with John McCain's wife, who is -- first time ever -- is endorsing me because of what he talks about with my son and (John), who are heroes, who served their country. You know he said they're losers, they're suckers," Biden said.

Representatives for Cindy McCain did not immediately respond to a request for comment.

The high-profile endorsement from the widow of the 2008 Republican presidential nominee comes as Biden attempts to peel off independents and Republicans alienated by Trump's comments and actions -- including in Arizona, which has emerged as a major electoral battleground.

Trump has frequently attacked John McCain by name, including in 2015, when he said he didn't consider McCain a war hero because he preferred those who were not captured. He again went after the late senator Wednesday morning, criticizing him for "endless wars" and the state of the Veterans Administration.

"Biden was John McCain's lapdog," Trump tweeted.

In a video at this year's Democratic convention, Cindy McCain was featured. "My husband and Vice President Biden enjoyed a 30+ year friendship dating back to before their years serving together in the Senate, so I was honored to accept the invitation from the Biden campaign to participate in a video celebrating their relationship," she said in a tweet about the video.

This story and headline have been updated with McCain's endorsement.