There’s good reason for that concern: Trump’s tribal, racial appeal threatens the GOP in both the near and long term. Yet it’s also understandable that Trump seemed blindsided by the heated Republican reaction to his attacks on the Indiana-born Curiel as a “Mexican” who cannot judge him impartially—and his indication on Face the Nation that he might not get a fair hearing from a Muslim judge either. (It’s reasonable to ask: would a President Trump demand that Supreme Court Justice Sonia Sotomayor recuse herself from all cases involving his administration because of her Hispanic heritage?*)

Trump has reason to be surprised because until now, Republican leaders have mustered no more resistance to his provocations than momentary grumbling, followed by capitulation. Trump personally demeaned Marco Rubio during the campaign as contemptuously as one presidential candidate has ever belittled another; yet Rubio compliantly endorsed him. Ryan criticized Trump over his Duke remarks and his proposal to temporarily ban Muslim immigrants. But then, after briefly withholding his endorsement, he too fell into line (if perhaps only temporarily).

Throughout, even the Republican leaders most uneasy with Trump have recoiled from confronting him partly because he has demonstrated how much of the GOP coalition responds to a racially barbed message of defensive nationalism. Trump’s appeal extends beyond racial backlash: His economic message and identity as an outsider and business executive also powered his victory. But there’s no question he has drawn his greatest support from the GOP voters most uneasy about demographic and cultural change. As Pew Research Center polling shows, Republicans who say the growing number of immigrants threaten traditional American values, and those who believe Islam is more likely to encourage violence, rate Trump far more favorably than those who reject those statements. Likewise, while voters who support deporting all undocumented Mexican immigrants only represented a minority of all GOP primary voters in almost every state with an exit poll, those deportation supporters backed Trump in such overwhelming numbers that they provided a majority of his votes almost everywhere. Meanwhile, polls consistently show broad majority support among Republican voters for Trump’s temporary ban on Muslims entering the country.

Republican leaders understandably have thought twice, or more, about confronting so much of their core coalition, particularly the non-college-educated whites who embrace those positions in largest numbers and keyed Trump’s nomination. Yet Trump’s swerve into more unvarnished racial arguments against Curiel has crystallized the risk in allowing him to define the party unchallenged. Polls indicate that most Americans oppose Trump’s signature proposals to ban Muslim immigration or deport 12 million undocumented immigrants. The raw racial tribalism of his attacks on Curiel will likely provoke even greater resistance.