Were Jimmy Carter and Joe Biden friends?

When President Joe Biden addressed the nation Sunday night to reflect on the loss of former President Jimmy Carter, he started by saying he lost a "dear friend."

Biden went on to say that he and Carter had "hung out" for the last 50 years, explaining that their families were bound by something deep: cancer.

In 2015, Jimmy Carter revealed he had surgery to remove a mass on his liver. About a week later, the then-90-year-old announced he had been diagnosed with cancer. Soon after that, he said the cancer had spread to his brain. He told the media he thought he only had a few weeks left to live. He, of course, went on to live another 10 years.

Unfortunately, former Delaware Attorney General Beau Biden died from brain cancer later at age 46 that year.

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"Jimmy Carter was just as courageous in his battle against cancer as he was in everything in his life. Cancer was a common bond between our two families, as in many other families," he said. "And our son Beau died – when he died, Jimmy and Rosalynn were there to help us heal."

Biden said he and his wife, first lady Dr. Jill Biden, also comforted the Carter family during Jimmy's battle.

Biden endorses Carter for president in 1976

In 1976, Joe Biden, who was a senator at the time, actually gave Jimmy Carter his first presidential endorsement from a national figure.

Biden said what sold him was Carter's character.

Presidential candidate Jimmy Carter accepts the support of Senator Joseph Biden, D-Del, at a news conference in the Sheraton Hotel in Madison.

"When I endorsed him for president, I told him why I was endorsing him. It was not only his policies, but his character, his decency, the honor he communicated to everyone," Biden explained. "He was also very, very … he and Rosalynn were very kind to Jill and me when we lost our son Beau. And, I don't know, I think that what Jimmy Carter is an example of is just simple decency."

Carter endorses Biden for president in 2020

The Carter family returned the favor when Biden ran for president in 2020.

"It's a great pleasure for Jimmy and me to join you in celebrating our next President of the United States, Joe Biden," Rosalynn said in a video played at the Democratic National Convention that summer. "Jimmy and I are voting for Joe because he recognizes the challenges facing our families and has the heart and the talent to make life better for all Americans."

"When I ran for president in 1976, Joe Biden was my first and most effective supporter in the senate for decades. He has been my loyal and dedicated friend. Joe has the experience, character and decency to bring us together and restore America's greatness," Jimmy Carter said in the same video.

NBC NEWS -- 2008 Democratic National Convention: Day 2 -- Air Date 08/25/2008 -- Pictured: (l-r) Vice Presidential nominee Senator Joe Biden (D-DE), Former President Jimmy Carter and (future First Lady) Dr Jill Biden in the audience at the Pepsi Cent

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Joe Biden says he misses Jimmy, Rosalynn

Toward the end of Biden's statement Sunday evening, he said that although he was sad, he found solace in knowing that his friend Jimmy was reunited with the love of his life.

"I will miss them both dearly."