Updated at 12:37 p.m. ET

Cindy McCain, the widow of Sen. John McCain, is the latest prominent conservative to urge Republicans to cross party lines and support Joe Biden for president.

"There's only one candidate in this race who stands up for our values as a nation, and that is Joe Biden," she tweeted Tuesday evening.

McCain added: "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. 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."

McCain's endorsement comes nearly a month after she contributed to a video that aired during the Democratic National Convention that detailed the friendship between her late husband and the Democratic presidential nominee.

Biden had scooped McCain's news several hours in advance of her announcement, telling supporters during a virtual fundraiser that McCain had decided to endorse him after The Atlantic reported that President Trump made disparaging comments about fallen U.S. service members.

Trump also has long been critical of McCain specifically.

McCain told NPR's Mary Louise Kelly on Wednesday that there were many reasons factoring into her choice to endorse Biden, including Trump's alleged comments on the military.

"I think most of us ... when we heard or observed the comments that were made about our troops, it was just unbelievable. And I was in disbelief with it for a long time."

McCain's late husband was a naval aviator shot down over Vietnam and imprisoned and tortured. In an earlier era, the Republican Party built a brand around his record of service for the 2008 presidential race, which McCain lost to President Barack Obama.

By 2015, Trump was criticizing McCain: "I like people who weren't captured." Republicans went on to nominate Trump the following year.

McCain told NPR she believes the current moment calls for putting country above party.

"This isn't about Republican versus Democrat in my mind. It transcends that. It transcends everything that has occurred. And certainly 2020 is the most unusual year we can name to date. So we need someone that can take the helm in a time of crisis, which is where we're at right now, and someone that not only supports our troops but believes in the good of America and not the ill of America."

McCain said she thought her late husband would be disappointed with the lack of civility and the vitriol in politics today.

"I think he would be supportive of me in doing what I think is right. He was always supportive of me. And as you know, my husband and I did not agree on everything ... whether or not he would have done this, I don't know. But I believe that the ideals that John possessed are the same ideals as Joe Biden. And I've got to believe that he's somehow watching all of this right now."

Arizona implications

Supporters hope Cindy McCain's endorsement could boost Biden's prospects in the critical state of Arizona and carry weight among Republicans who are disillusioned with Trump but unsure of casting their ballot for Biden.

"I'm hoping that I can encourage suburban women to take another look, women that are particularly on the fence and are unhappy with what's going on right now but also are not sure they want to cross the line and vote for Joe. I hope they'll take a look at what I believe and will move forward and come with me and join team Biden," she said.

McCain told NPR's Kelly she plans to campaign for Biden in virtual events and in person should Biden travel to Arizona to campaign.

Trump responded to McCain's endorsement in a tweet Wednesday morning, writing: "Joe Biden was John McCain's lapdog. So many BAD decisions on Endless Wars & the V.A., which I brought from a horror show to HIGH APPROVAL. Never a fan of John. Cindy can have Sleepy Joe!"

McCain said the president's numerous tweets about her late husband and family don't get under her skin.

"I don't follow him and I don't pay attention to what's going on at all," she said.

McCain's daughter Meghan, co-host of The View, has been a vocal critic of Trump for both his brand of politics and his personal attacks on her family. She delivered a powerful eulogy of her father in 2018, rebuking Trump himself — who was not invited.

Meghan McCain recently told Watch What Happens Livehost Andy Cohen that it doesn't take "a rocket scientist" to figure out who she is voting for.

"There's one man who has made pain in my life a living hell and another man who has literally shepherded me through the grief process," she said.

Copyright 2021 NPR. To see more, visit https://www.npr.org.