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Why Joe Biden got choked up over this critic

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The emotion in Joe Biden's voice makes it clear there's more than one level to his relationship with former Defense Secretary Robert Gates.

Gates, who had clashed professionally with Biden, made headlines when his memoir two years ago asserted that the vice president had been "wrong on nearly every major foreign policy and national security issue over the past four decades." But their personal relations are different.

Joe Biden speaks with CNBC's John Harwood.
Sophie Bearman | CNBC

"He sent me the most incredibly thoughtful letter when my son died, which really mattered," Biden said, his voice cracking during an interview with CNBC. After pausing to compose himself, Biden added, "He talked about what kind of father he thought I was. He talked about the sacrifice Beau had made, when he knew he didn't have to go to Iraq. And he talked about what kind of man Beau was. He knew him.

"Everybody wants to be nice, and they were," the vice president went on. "But this was so pointed and personal that I called him, and wrote him. And matter of fact, I wrote him another letter not long ago when I was going over some mail, just to tell him how much it meant."

Joe Biden speaks with CNBC's John Harwood.
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Not that it settled their policy disagreements. On the Gates book, Biden said: "What I said to Bob (was), 'Bob, I'm delighted to debate you. I was right about Gorbachev, you were wrong. I was right about Vietnam, you were wrong. I was right about it. We just happen to disagree.'

"That was about substance," Biden said. "I'm delighted to debate Bob on who was right and wrong."

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