President Donald Trump just won’t let it go. Maggie Haberman and Jonathan Martin report for the New York Times that Trump is still promoting conspiracy theories about whether former President Barack Obama was born in the US:

In recent months, [advisers] say, Mr. Trump has used closed-door conversations to question the authenticity of President Barack Obama’s birth certificate. He has also repeatedly claimed that he lost the popular vote last year because of widespread voter fraud, according to advisers and lawmakers. One senator who listened as the president revived his doubts about Mr. Obama’s birth certificate chuckled on Tuesday as he recalled the conversation. The president, he said, has had a hard time letting go of his claim that Mr. Obama was not born in the United States. The senator asked not to be named to discuss private conversations.

That comes from a story largely about Trump’s penchant for conspiracy theories. The story also highlights that Trump has been questioning whether the Access Hollywood tape — in which he’s heard saying that he can “grab [women] by the pussy” and get away with it because he’s a celebrity — is authentic, even though he previously acknowledged that the tape was legit.

But touting the idea that Obama wasn’t born in the US isn’t new for Trump. He essentially launched his current political career calling by becoming one of the most prominent advocates for the “birther” conspiracy theory. He continued spouting off these conspiracy theories in public as late as 2015 — years after Obama released short-form and long-form birth certificates proving he was born in Hawaii. (Although in 2016 Trump finally said that “President Barack Obama was born in the United States.”)

Continuing to do this as president, though, is another thing. It’s another way that Trump — who just this week used a racially loaded language (“Pocahontas”) to mock a sitting US senator — has leveraged his seat in the Oval Office to make ridiculous, offensive comments.

It’s also a naked appeal to racist attitudes. It’s no coincidence that the first president to face major, persistent questions about whether he was born in the US is black and has a foreign-sounding name. Studies have shown that a key driver of birtherism is racial resentment.

A key driver of birtherism: racial resentment

Political scientist Philip Klinkner of Hamilton College examined the effects of race and racial attitudes on birtherism in a 2014 paper. He concluded that belief in birtherism “is almost completely resistant to factual correction and is strongly related [to] partisanship and attitudes about race.”

To evaluate this, Klinkner conducted a survey of Americans asking them about their views on Obama’s birthplace. He then compared the answers to other factors, such as political party, race, and racial attitudes. He found that birthers are almost entirely white, are mostly Republican, and reported high levels of racial resentment.

In fact, a stronger belief in birtherism correlated tightly with increasing levels of racial resentment.

It’s possible the correlation was coincidental. The study acknowledged that whiteness, Republicanism, and racial resentment all tend to correlate, so maybe this really reflects that partisan beliefs, not racial resentment, drive birtherism. But when Klinkner put all of these factors through a statistical control model, he found that racial resentment significantly correlated by itself with birtherism.

To prove this, Klinkner also looked at Democrats who believed Obama was born outside the US. He found, “Among those with the lowest levels of racial resentment, party had little influence as both Democrats and Republicans had a low probability of believing in birtherism. As racial resentment increased, however, the probability of birtherism increased for both Democrats and Republicans, but more among the latter.” So partisan beliefs did play some role, but racial resentment played a significant role as well.

This is what Trump is appealing to when he continues spouting off conspiracy theories about Obama’s birth certificate. And he’s now reportedly doing it from the White House.