Bernie Sanders wants to see Trump lose in 2020

US Sen. Bernie Sanders, I-Vt in Tampa, Fla.

(CNN)Sen. Bernie Sanders "wants to make sure that Trump is beaten in 2020," according to his 2016 campaign manager, Jeff Weaver.

Weaver, the author of "How Bernie Won: Inside the Revolution That's Taking Back Our Country -- and Where We Go from Here," said that Sanders, a Vermont independent, will run in 2020 only if "he's the person who's best able to do that."
And what does Sanders' former campaign manager think?
"I'm very clear about what I think he should do," Weaver told David Axelrod on "The Axe Files," a podcast from The University of Chicago Institute of Politics and CNN.
    The last three words of his book are "Run, Bernie, run."
    Weaver first met Sanders after being asked to leave Boston University, where he says he became "politicized" after joining the Soviet Jewry movement and the anti-apartheid movement on campus. He moved back to his hometown, St. Albans, Vermont.
    "I had been sort of radicalized at undergraduate school, and I came back home again to this sort of sleepy place. ... I'm kicked out of college. I don't have a job."
    Weaver met Sanders at an event he was staffing. Soon after, he received a call from Sanders, then the mayor of Burlington, asking him to work for Sanders in Burlington. Thirty years later, when Sanders decided to run for president, he and Weaver were still colleagues.
    Here are three things we learned about Sen. Bernie Sanders from his 2016 presidential campaign manager, Jeff Weaver.

    1. Sanders believes in the power of talking to people

    "He tries to talk to more people about the vision he articulates, and he's confident that if he can do that with enough people, he'll be successful."
    Weaver says he observed this in Sanders when they visited Iowa.
    "He would hold rallies and he would get the rolling track of how many actual people attended the rallies, just because he wanted to know what percent of the eventual electorate he actually talked to, basically, in person."
    Weaver and the rest of Sanders' team would remind the candidate that there was a broader audience watching him on television, but he seemed to care more about getting face time with potential voters.

    2. Being president wasn't 'personal for him'

    "As he often tells me, it was like, '... I wasn't somebody who was born and groomed to be president of the United States. I don't have to be president of the United States.' "
    Weaver said Sanders wanted to win "on the basis of the ideas that he was putting forward" rather than focusing on his opponents' personalities or personal lives.
    "He's not somebody who was groomed from a child to be the next leader of the free world."

    3. Sanders kept his early campaigns low-key

    "Bernie did a lot of the 'running' of the campaign," said Weaver, who also had worked with Sanders in 1986, 1988 and 1990. "It was very few people."
    This approach worked to Sanders' advantage when he was the lesser-known mayor of Burlington, Vermont.
    "He would meet with small groups of people -- eight, 10 people -- a far cry from the 30,000-person crowds he had in 2016."
      Sanders wasn't really on the radar for people outside of Burlington, according to Weaver, which gave the senator "a lot of work to do." The work paid off.
      "He ended up being very, very popular with rural people in particular, I think, who appreciated his sincerity and understood that he believed what he said."