Former President Barack Obama is keeping quiet about the Democratic presidential primary, but some of his opinions have come to light anyway, thanks to a new report from Politico. And there are at least two frontrunners who he might not want to see snag the nomination.

Sen. Bernie Sanders (I-Vt.) especially doesn't appear to have a big fan in Obama. Per Politico, when Sanders seemed to have a greater chance of winning (though some disagree with the characterization that he's trending downward), Obama privately said he would speak up to stop Sanders if he was running away with the nomination.

It's unclear if Obama stills feels this way — a spokesperson for Obama noted the former president said he would support and campaign for whoever the party's nominee is. But a closer adviser, while not confirming Obama's comments, added that "if Bernie were running away with it, I think maybe we would all have to say something."

Sen. Elizabeth Warren (D-Mass.), meanwhile, also has a complicated relationship with Obama, Politico reports. While there's no indication Obama would speak out against Warren, he reportedly did once say privately in 2015 (when Warren was thinking about jumping into the 2016 presidential race) that Democrats rallying around her would be a "repudiation" of him and his economic policies. Read more at Politico. Tim O'Donnell