Former President Barack Obama has been privately praising presidential hopeful Sen. Elizabeth Warren (D-Mass.), The Hill reported Monday.

According to sources familiar with the interactions, Obama has been talking up Warren in conversations with donors who are hesitant to support a candidate running a campaign that’s critical of Wall Street and America’s billionaire class.

“He obviously thinks she’s very smart,” one Democratic donor told The Hill. “He thinks her policy ideas matter. And I think he sees her running the campaign with the most depth.”

A longtime Obama ally told The Hill that Warren “checked the box” for all of the criteria Obama considers important in a presidential candidate. “I think he feels licensed to give an opinion on her because he’s ‘hired’ her,” the person added.

Spokespeople for Obama and for Warren’s campaign did not immediately respond to HuffPost’s requests for comment on the matter.

Obama, whose onetime running mate, former Vice President Joe Biden, is also vying for the Democratic nomination, has declined to formally endorse any of the candidates. Julián Castro, who served as Obama’s head of the Housing and Urban Development Department, is another one of candidates hoping to unseat President Donald Trump in 2020.

Sen. Elizabeth Warren (D-Mass.) served as a special adviser on former President Barack Obama's Consumer Financial Protection Bureau. (Photo: Kevin Lamarque / Reuters)

Privately, however, Obama has reportedly vowed to speak out against Sen. Bernie Sanders (I-Vt.) if he is on track to become the nominee.

Support from Obama could be a boon for Warren, who’s trailing just behind South Bend, Indiana, Mayor Pete Buttigieg in polls for the Iowa caucus that’s less than two months away.

She already has support from more than 200 alumni of Obama’s campaigns and administration who signed on to a letter organized by a pair of former senior Obama aides.

There was periodic tension between Obama and Warren throughout his presidency. As a Harvard Law professor, Warren spearheaded the formation of Obama’s Consumer Financial Protection Bureau, but sparred with him over what role she could play in it ― reportedly causing a rift with both Obama and swaths of his staffers.

Warren doesn’t say much about Obama on the campaign trail, but what she does say is generally positive. However, her vows to tackle poverty by breaking up big businesses and raising taxes on the wealthy have the same undercurrent of one of her biggest gripes with the Obama administration. His top-down approach to avoiding a depression and global financial crisis by saving the country’s biggest financial institutions instead of starting with Americans losing their homes, she’s argued, is a big part of what helped someone like Trump ascend to the White House.

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This article originally appeared on HuffPost.