Could Bernie Sanders run against Elizabeth Warren in 2020 presidential race? Sanders' former campaign manager asks: "Who can beat Trump in 2020?"

Sen. Bernie Sanders is still considering a second run for president, according to his former campaign manager Jeff Weaver, and he may decide to enter the race even if it means running against another progressive powerhouse, Massachusetts Sen. Elizabeth Warren.

Warren has also been on Democrats’ short list for the 2020 presidential race to challenge President Trump, and Sanders’ longtime aide dodged questions about whether the two New England senators would really go head-to-head.

“The question is who can beat Trump in 2020, how we can put together the type of coalition from the folks who have taken it over and I think that is what the senator is weighing,” Weaver said during an interview with ABC News.

“I am confident that whoever runs there will be progressive standard-bearer who will be able to galvanize this grassroots energy.”

Weaver added, “Whoever wants to run for president should run."

“I think, the more the merrier,” he said.

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Weaver sat down with ABC News to discuss his new book, “How Bernie Won,” in which he urges the Independent Vermont senator to run again in 2020 after detailing the launch and unexpected success of Sanders’ campaign from his perspective.

He argues in the book that the Democratic PArty, as a result of Bill Clinton-era more conservative and centrists policies, had gotten off course and that now, policy positions such as a $15 minimum wage, debt-free public college, and single-payer health care have taken off within the Democratic Party as a result of the senator’s campaign.

Weaver said during the interview that he is not simply interested in “party purity,” but that those progressive policy positions were also smart politics. He posited that Sanders won, what he called, “the geography primary,” beating Hillary Clinton in states in the middle of the country where Democrats have been struggling and that are typically thought to be more conservative.

In his book, Sanders also recounts times his team felt the Democratic Party was actively working against Sanders campaign. He dismisses arguments that Sanders’ run could have hurt Hillary Clinton’s chances, and states repeatedly that the senator would have beaten President Trump in a general election two years ago.

“To the extent our campaign bears any responsibility for Trump’s victory it is that we did not defeat Hillary Clinton,” he writes in the book.

“The polling was consistent that Bernie Sanders was the stronger candidate against virtually every Republican candidate and certainly against Trump,” Weaver told ABC News.

“As an empirical matter,” he added, “She did not win and so Bernie Sanders’s chances of winning empirically were as good as hers or better.”