In public, President Donald Trump has fixated on mocking Sen. Bernie Sanders (I-VT) as “crazy” and has focused on assuring supporters of how badly he’d crush the left-wing senator in a general election and protect them from Sanders’ creeping socialism.

In private, Trump has often expressed different worries about the 2020 candidate.

In the past two months, the president has repeatedly asked advisers, in and out of the White House, about how Sanders polls, or would likely perform, in critical battleground states, specifically Pennsylvania, according to two people who’ve independently heard him ask about it. At times, Trump has asked about Sanders’ prospects even in the absence of a current public or internal poll on the matter.

It’s hardly the first time Trump has talked to his friends and top lieutenants about Sanders’ potential formidability heading into an election year. As The Daily Beast reported last year, Trump had on multiple occasions told longtime associates and Republican donors that running against “socialism” in 2020 may not be “so easy” due to its populist appeal. At a private event with donors late last year, the president said, according to a source in the room, that “you can have someone who loves Trump, but many people love free stuff, too,” adding that if Sanders or other “socialist” candidates promise U.S. voters—particularly young voters—that they will cancel their debt, “that’s a tough one [to run against].”

But at Tuesday night’s rally in Milwaukee, Wisconsin, the president stuck to the crowd-pleasers when it came to Sanders. “Bernie is surging, Bernie is surging,” Trump said, eliciting lusty boos from the audience. In several recent surveys, including a Des Moines Register poll, one of the few released out of the first caucus state in the lead-up to the Feb. 3 event, Sanders came out on top. In New Hampshire, he’s been a leading neighboring state contender for weeks.

Trump also couldn’t resist referencing Sen. Elizabeth Warren’s (D-MA) claim this week that Sanders told her a woman couldn’t defeat Trump in 2020.

“I don’t believe that Bernie said that,” said Trump.

The Sanders campaign has long had their candidate polling ahead of Trump. According to data from an internal April 2019 poll obtained by The Daily Beast, the last time the campaign said they tested in Pennsylvania, the senator was winning in the state by 8 points at 51 percent to Trump’s 43 percent.

That lead has dwindled over time, and now according to the Real Clear Politics polling average, is just under 4 percent, half of the lead former Vice President Joe Biden enjoys over the president, according to the same average.

Still, those numbers haven’t diminished Trump’s concerns about the senator.

Trump’s team has repeatedly declined to share their internal polling data with reporters and the public. Indeed, the president instructed his aides last year to tell reporters that his campaign’s internal numbers didn’t show him polling behind Biden in critical states—even though that same internal polling had him trailing Biden, then the clear national Democratic frontrunner.

Still, taken together, Sanders’ top aides and outside surrogates still believe the durability of his figures, and his recent traction in the Democratic primary, can help poke holes in Trump’s electability argument, portraying him as dishonest, especially to workers’ best interests.

Trump’s senior staff is, of course, reliably projecting calm at the thought of running against Team Bernie.

“We have no preference among the Democrat candidates because President Trump would beat any of them handily,” said Tim Murtaugh, communications director for Trump’s re-election campaign. “Over the last year, the campaign has engaged various Democrats when the opportunities have presented themselves. In Sanders’ case, his surge in the polls coincided with his emergence as the chief apologist for the Iranian regime. We needed to point out that he would be dangerous as president since he made clear he would appease terrorists and terror-sponsoring nations.”

Still, no one in Sanders-world interviewed by The Daily Beast said they were surprised that Trump has been privately expressing concerns about the appeal of Sanders and the senator’s brand of Democratic socialism, particularly in several states he won.

“I’ve always thought he would be obsessed with the senator,” Nina Turner, a senior Sanders adviser and his campaign’s national co-chair, told The Daily Beast about Trump. “He knows that he is the true progressive and the right person to go up against his faux populism.”

Sanders’ communications director Mike Casca put it another way: “They should be very worried.”

Others in Sanders’ circle point to his strength with working class voters, in part, who first voted for him in 2016 and swung to Trump in the general election. Multiple sources said they have worked to expand that base of support in the 2020 cycle.

“Bernie’s appeal,” Jim Zogby, a national surrogate for Sanders, said, “is to multi-ethnic and white working-class voters across the board. That is the crowd that the [Democratic] Party handed to Trump on a platter and Bernie is in effect saying, ‘Thank you, I’ll take that.’”