Former President Barack Obama has not endorsed a Democratic presidential primary candidate yet, but one adviser told Politico that Obama has said there's only one candidate he would publicly oppose in the fight for the nomination — Sen. Bernie Sanders (I-Vt.).

Obama, who is still extremely popular among Democratic voters, has avoided directly influencing the Democratic primary. He hasn't even publicly supported Joe Biden, his former vice president during his two terms (although Biden claims he has asked Obama not to endorse him).

According to those close to him, and his own public remarks, Obama has no intention of affecting the race by throwing his support behind a specific candidate. But, if Sanders gained momentum and began to look like the presumptive nominee, Obama might be compelled to speak out.

As Ryan Lizza wrote for a Politico Magazine peace titled, "Waiting for Obama":

Publicly, he has been clear that he won't intervene in the primary for or against a candidate, unless he believed there was some egregious attack. "I can't even imagine with this field how bad it would have to be for him to say something," said a close adviser. Instead, he sees his role as providing guardrails to keep the process from getting too ugly and to unite the party when the nominee is clear. There is one potential exception: 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. (Asked about that, a spokesperson for Obama pointed out that Obama recently said he would support and campaign for whoever the Democratic nominee is.)

Another adviser told Lizza that he could not confirm Obama's stance on Sanders directly, but that "we would all" have to speak out if the Democratic socialist were running away with the nomination. A Sanders landslide is, at this point, seemingly unlikely.

"Bernie's not a Democrat," one Obama friend told Politico.

Obama took a slow approach to the 2018 election as well, mostly staying out of it until the fall of 2018 when he was actively campaigning for candidates across the country. While that may have helped in some cases, there were some high-profile disappointments.