“I’m against the anchor babies, and I’m against the Muslims.” That’s something a Donald Trump supporter named Kathy Parker told me at a Donald Trump rally I reported from recently. It’s an attitude that helps explain Trump’s call on Monday to ban all Muslim immigration to the U.S.—and the split he has exposed in the base of the Republican Party.

Prior to Monday’s announcement, it was already abundantly clear what Trump was getting at with his explicitly anti-Muslim and anti-immigrant rhetoric. At the rally I attended in Myrtle Beach, which drew more than 10,000 people—and which occurred prior to last week’s shooting in San Bernardino—I met plenty of people like Parker, who had driven eight hours, with several family members, to attend the event. In his speech that night, Trump said, “Radical Islamic terrorists. Obama refuses to use the word. He refuses to say it, he can’t say it. There’s something going on, I don’t know what it is.” Several attendees could be heard to shout, “He’s a Muslim!” or “He’s one of them!”

So Trump was already openly preaching paranoid nativism. But by putting it in writing, as a formal policy proposal, he has upped the ante. This isn’t an implication, a tossed-off musing, or a dog whistle anymore; this is what he is running on.

These new developments cast in stark terms the dilemma Trump poses for the Republican Party. His positions have drawn condemnation from such GOP figures as Dick Cheney: “This whole notion that somehow we can just say no more Muslims, just ban a whole religion, goes against everything we stand for and believe in.” And House Speaker Paul Ryan: “This is not conservatism. What was proposed yesterday is not what this party stands for.” Yet 30 percent of Iowa Republicans said in a recent poll they think Islam should be outlawed. In another survey, 76 percent of Republicans—and 56 percent of all voters—said they considered the values of Islam “at odds with American values and way of life.”

Back in August, the conservative writer Ben Domenech asked, in a prescient essay, “Are Republicans for freedom or white identity politics?” Trump, he said, threatened to reorient the GOP away from ideological conservatism, along the lines of right-wing European political movements. The divide within the GOP has long been described as the “establishment”—power brokers, donors, elected officials, consultants—versus the “conservative base.” But it’s increasingly clear there are two separate conservative bases within the GOP.

There’s the intellectual conservative movement, a decades-long project of institutional actors like the Heritage Foundation and the American Conservative Union, which seeks to push the party toward strict adherence with a set of ideas about limited government, strong national defense, and the traditional family. And then there is the populist, nativist strain, which isn’t really about ideas so much as a raw appeal to emotion. Trump’s dominance of the primary field is forcing the party to confront a frightening prospect: that the populist bloc may be the bigger of the two.