Charlie Neibergall, AP
FILE - In this Dec. 19, 2015, file photo, Republican presidential candidate Donald Trump speaks in Cedar Rapids, Iowa. Americans place a higher priority on preserving the religious freedom of Christians than other faith groups, ranking Muslims as the least deserving of the protections, according to a new poll from The Associated Press-NORC Center for Public Affairs Research. The poll was conducted Dec. 10 through Dec. 13, after Islamic extremist attacks in Paris and San Bernardino, California, and during intensifying anti-Muslim rhetoric by Trump and other candidates for the Republican presidential nomination. (AP Photo/Charlie Neibergall)

WASHINGTON — Every Republican of the type concerned with winning in November has been asking the question (at least internally), "What if the worst happens?"

The worst does not mean the nomination of Ted Cruz, in spite of justified fears of political disaster. Cruz is an ideologue with a message perfectly tuned for a relatively small minority of the electorate. Uniquely in American politics, he has made his reputation by being roundly hated by his colleagues — apparently a prerequisite for a certain kind of anti-establishment conservative, but unpromising for an image makeover at his convention.

Cruz's nomination would represent the victory of the hard right — religious right and tea party factions — within the Republican coalition. After he loses, the ideological struggles within the GOP would go on.

No, the worst outcome for the party would be the nomination of Donald Trump. It is impossible to predict where the political contest between Trump and Hillary Clinton would end up. Clinton has manifestly poor political skills and Trump possesses a serious talent for the low blow. But Trump's nomination would not be the temporary victory of one of the GOP's ideological factions. It would involve the replacement of the humane ideal at the center of the party and its history. If Trump were the nominee, the GOP would cease to be.

Whatever your view of Republican politicians, the aspiration, the self-conception, of the party was set by Abraham Lincoln: human dignity, honored by human freedom and undergirded by certain moral commitments, including compassion and tolerance. Lincoln described the "promise that in due time the weights should be lifted from the shoulders of all men, and that all should have an equal chance."

It is this universality that Trump attacks. All of his angry resentment against invading Hispanics and Muslims adds up to a kind of ethno-nationalism — an assertion that America is being weakened and adulterated by the other. This is consistent with European, right-wing, anti-immigrant populism. It is not consistent with conservatism, which, at the very least, involves respect for institutions and a commitment to reasoned, incremental change. And Trumpism is certainly not consistent with the Republicanism of Lincoln, who admitted no exceptions to the promises of the Declaration of Independence and was nominated, in part, because he could appeal to anti-slavery German immigrants.

Liberals who claim that Trumpism is the natural outgrowth, or logical conclusion, of conservatism or Republicanism are simply wrong. Edmund Burke is not the grandfather of Nigel Farage. Lincoln is not even the distant relative of Trump.

Trump, in some ways, is an odd carrier of ethno-nationalist beliefs. He held few of them, as far as I can tell, just four years ago. But as a demagogue, he has followed some of America's worst instincts wherever they have led, and fed ethnic and religious prejudice in the process. All presidential nominees, to some extent, shape their parties into their own image. Trump would deface the GOP beyond recognition.

Trump is disqualified for the presidency by his erratic temperament, his ignorance about public affairs and his scary sympathy for authoritarianism. But for me, and I suspect for many, the largest problem is that Trump would make the GOP the party of racial and religious exclusion.

American political parties are durable constructions. But they have been broken before by powerful, roiling issues such as immigration and racial prejudice. Many Republicans could not vote for Trump but would have a horribly difficult time voting for Clinton. The humane values of Republicanism would need to find a temporary home, which would necessitate the creation of a third party. This might help elect Clinton, but it would preserve something of conservatism, held in trust, in the hope of better days.

Ultimately, these political matters are quite personal. I have spent 25 years in the company of compassionate conservatives, reform conservatives, Sam's Club conservatives, or whatever they want to call themselves, trying to advance an agenda of social justice in America's center-right party. We have shared a belief that sound public policy — promoting opportunity, along with the skills and values necessary to grasp it — can improve the lives of our fellow citizens, and thus make politics an honorable adventure.

The nomination of Trump would reduce Republican politics — at the presidential level — to an enterprise of squalid prejudice. And many Republicans could not follow, precisely because they are Republicans. By seizing the GOP, Trump would break it to pieces.

Michael Gerson's email address is michaelgerson@washpost.com.