Kerry also talks with Brennan about his role in politics today, the current political environment, his relationship with the late Sen. John McCain, and his wife, Teresa Heinz Kerry.

Now in private life, the former Massachusetts senator and Secretary of State under President Obama has watched as much of his political legacy – including the Iran nuclear deal and the climate change accord – has been upended under the Trump administration. Disappointment in those actions has motivated Kerry to attempt to help Democrats regain political control.

Will he run for president in 2020?

"Talking about 2020 right now is a total distraction and waste of time," Kerry told Brennan. "What we need to do is focus on 2018. We need to win back the confidence of the country to move in a better direction, and to do it in sensible ways."

He added, "We are in a moment of crisis for our country. The world is in a moment of crisis."

Kerry tells Brennan sitting on the sidelines is not possible. But what does that mean for his future? "I'm going to continue to be an activist," he said. "I'm going to continue to fight."

Kerry also shares regrets about his 2004 bid for the White House, including the choice of then-Sen. John Edwards as his running mate. [Edwards was later caught up in a sex scandal.] 

Does he regret the decision today? "If you measure the qualities that you are looking for when you choose a vice president," he said, "it didn't measure up. It wasn't what we had hoped for."

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