Though he said he'd remain neutral during the Democratic primary, Barack Obama reportedly vowed to stop one candidate from securing the party's nomination. 

According to a newly published Politico report, the former president privately told advisers he "would speak up to stop" Bernie Sanders from becoming the nominee, if the Vermont senator was significantly leading the race. A spokesperson for Obama could not confirm nor deny the report, but suggested a Sanders nomination seemed unlikely at this point.

Per Ryan Lizza of Politico: 

When it comes to Sanders, I asked one close adviser whether Obama would really lay himself on the line to prevent a Sanders nomination. “I can’t really confirm that,” the adviser said. “He hasn’t said that directly to me. The only reason I'm hesitating at all is because, yeah, if Bernie were running away with it, I think maybe we would all have to say something. But I don't think that's likely. It's not happening.” (Another close Obama friend said, “Bernie's not a Democrat.”)

Sen. Sanders has remained a front-runner throughout the primary season and has become the fastest candidate in history to reach 4 million individual donations. Sanders' progressive proposals—such as Medicare for All, student loan cancelation, and free college tuition—have received pushback from a number of centrist Democrats who say his policies are "too far left" for Americans. Obama has echoed this sentiment, recently warning 2020 candidates about straying too far from the center.

"Even as we push the envelope and we are bold in our vision we also have to be rooted in reality," Obama said earlier this month while addressing the Democracy Alliance. "The average American doesn’t think we have to completely tear down the system and remake it. They just don't want to see crazy stuff."

Obama didn't name names, of course, but his warning was seemingly directed at Sanders and Massachusetts Sen. Elizabeth Warren, another top-tier candidate who is also a favorite among self-identified progressives. Politico also addressed Obama and Warren's "complicated" relationship, as he reportedly told advisers that significant support for Warren would be seen as a rejection of his economic policies.

Obama said privately that if Democrats rallied around her as their nominee it would be a repudiation of him—a clear sign that his economic decisions after the Great Recession had been seen as inadequate. There are very few former senior Obama officials in Warren’s campaign.

Other highlights of the report include Obama's purported skepticism of Pete Buttigieg and Sen. Kamala Harris—doubting the former's ability to appeal to black voters. 

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