President Obama’s nomination of Merrick Garland to the Supreme Court Wednesday underscores how closely the Republican Party’s future influence over every lever of federal power is now bound to the volatile instincts of Donald J. Trump.

If Senate Republicans uphold their promise to block Garland, and Trump continues his march to the party’s presidential nomination, the GOP will be betting that Americans will choose to vest control of the next Supreme Court nominee in a candidate whose most recent judicial pronouncement was promising to explore paying the legal fees for an elderly white supporter who sucker punched a young black protester at one of his rallies. (After sending that unmistakable initial signal, Trump later retreated.)

It remains plausible that Trump will arrive at the GOP convention with fewer than the 1,237 delegates that he needs for a first-ballot nomination (though that remains within his reach.) But it’s no longer plausible that any rival will arrive there with as many delegates as him, and seems unlikely any will claim nearly as many—making him tough to deny even if he falls short.

In Tuesday’s contests, Trump demonstrated again that he has split the GOP along a new class-based axis. The core of Trump’s success remains his dominance among white voters without a college education across geographic, religious, and ideological lines. After Tuesday, exit polls have been conducted in 20 states. Trump has carried most non-college whites in 17 of them, often attracting about half their votes or more. Trump’s position with white-collar Republicans is much more equivocal. Across the 20 exit-polled states, he has carried most college-educated Republicans in just eight (and tied Ted Cruz in another). But Trump has generally remained more competitive with these voters than his opponents have with his blue-collar base.

The result has been a historic power shift. Usually when the GOP’s upscale and working-class wings have diverged, the white-collar favorite has prevailed (think Mitt Romney). But whites without a college degree have provided a majority of Trump’s votes in about three-fourths of the exit-polled states.

None of Trump’s remaining rivals has yet shown a broad enough reach to stop him. John Kasich relies too heavily on moderate voters, a minority within the party. In mirror-image, Cruz relies too much on the most conservative voters and evangelical Christians. Cruz has not yet carried even a plurality of voters who are not evangelical Christians in any exit-polled state (though he probably won them in some small caucus states.)

Trump critics hope he will fade once the race turns more toward “closed” contests that only allow Republicans to vote. But he has carried self-identified Republicans in every state he has won (except for Missouri, where he narrowly trailed among Republicans but narrowly leads overall at last count). Trump remains a plurality front-runner, whose total vote has exceeded 40 percent in only a few contests. Yet the evidence shows he has built his winning formula from within the heart of the Republican coalition, particularly among disaffected voters drawn to his bristling insular nationalism. One final pattern is telling: Although only a minority of Republican voters have supported deporting all undocumented immigrants in 14 of 16 states where the exit poll has asked that question, those pro-deportation voters have provided a majority of Trump’s votes in all 16 of those states.

But even as Trump advances toward the nomination, every light on the dashboard is blinking red about his general election competitiveness—notwithstanding likely Democratic nominee Hillary Clinton’s own manifest problems. Trump’s appeal to blue-collar whites could enable him to contest Rustbelt battlegrounds like Ohio, Iowa, Wisconsin, Michigan, and Pennsylvania, which have usually, or always, voted Democratic since 1992. But even before last weekend’s violence, his unfavorable ratings touched astronomical heights among Millennials, minorities, and college-educated white women. Each of those groups will almost certainly cast a larger share of the vote this year than in 2012, while the share cast by Trump’s white working-class base has declined in every election since 1980 except one. That leaves Trump facing daunting demographic headwinds.

The risks to the GOP if Trump wins its nomination but then falters are rapidly compounding. Senate races increasingly follow presidential results, and with Republicans defending seven Senate seats this year in states that twice backed Obama, a Trump defeat could easily cost the GOP the upper chamber. And that, in turn, would provide Democrats a free hand to reshape the Supreme Court—even if Senate Republicans maintain their blockade this year against Garland.

But the greatest risk, especially after Trump’s combative response to the violence surrounding his campaign, is that as the Republican nominee he could define it as the party of white backlash in the eyes of the growing Millennial and minority populations. “That’s the great danger—that he defines what the Republican Party is in the 21st century, and because of the demographic trends that is toxic,” said the long-time GOP strategist Whit Ayres, the chief pollster for Marco Rubio. “We are on a precipice here, particularly with the Hispanic folks. The danger is we could have Hispanic voters locked into the same voting patterns as African Americans. If that happens we will never elect another Republican president.”

Since announcing his candidacy, Donald Trump has repeatedly demonstrated that he will say or do almost anything. It increasingly appears Republicans this fall will be strapped onto that roller coaster for an election they have elevated into a national referendum on which president should set the Supreme Court’s balance, possibly for decades.