Former President Barack Obama , who has so far taken a neutral position publicly on the 2020 Democratic presidential primary, privately indicated he would speak up to stop Bernie Sanders from becoming the partys nominee should the Vermont senator make significant gains, Politico reported Tuesday.

Politicos Ryan Lizza, who spoke to several of Obamas advisers, wrote that the former president sees his role in the Democratic primary process as providing guardrails to make sure it doesnt get too ugly and to unite the party when the nominee is clear.

But theres one exception, Lizza wrote, and thats Sanders.

Back when Sanders seemed like more of a threat than he does now, Obama said privately that if Bernie were running away with the nomination, Obama would speak up to stop him, according to Lizzas report. Its unclear exactly what period of time the Politico reporter was referring to.

Sanders, who lost the Democratic primary race to Hillary Clinton in 2016, has remained among the front-runners since announcing his second presidential bid in February. A Quinnipiac University poll conducted nationally last month of likely Democratic voters showed Sanders at 15%, trailing behind Sen. Elizabeth Warren of Massachusetts at 28% and former Vice President Joe Biden at 21%.

Asked whether Obama would really step in to prevent a Sanders nomination, one adviser told Lizza that he cant really confirm that.

He hasnt said that directly to me, said the adviser, who was not identified. The only reason Im 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 dont think thats likely. Its not happening.

Another close Obama friend reportedly told Lizza that Bernies not a Democrat. (Sanders serves as an independent in the Senate, but caucuses with Democrats .)