Cindy McCain, wife of the late Sen. John McCain and a longtime Republican, will be among those highlighted on Tuesday night at the Democratic National Convention, where she will outline the "unlikely friendship" her husband had with former Vice President Joe Biden ahead of his nomination for the Democratic ticket.

"I was honored to accept the invitation from the Biden campaign to participate in a video celebrating their relationship," McCain said in a tweet about the two men's decadeslong friendship.

It's unclear whether her appearance on Tuesday will amount to an official endorsement of the former vice president.

tag