The outbreak of hostilities between Donald Trump and Ted Cruz may not be edifying, but it is clarifying.

Cruz represents the arrival of tea party ideology at the presidential level. He espouses a “constitutionalism” that would disqualify much of modern government, and a belief that Republican elites are badly, even mainly, at fault for accommodating cultural and economic liberalism. Trump has adopted an ethno-nationalism in which the constraints of “political correctness” are lifted to express frankly nativist sentiments: that many illegal immigrants are criminals and rapists who threaten American jobs, and that Muslims are foreign, suspicious and potentially dangerous.

These approaches can overlap, but they are not identical. Cruz is attacking Trump as a “fake conservative” on gun and property rights and as a New York liberal on cultural matters. For his part, Trump defends those portions of the welfare state that benefit the working class, opposing cuts in Social Security and an increase in the retirement age. Cruz is the conservative true believer. Trump is the wrecking ball of political convention. They are not only two strong personalities; they demonstrate two different tendencies within the right.

Trump’s attacks on Cruz have begun drawing both blood and protests from ideological conservatives. “Either cut the crap,” warns radio host Mark Levin, “your accusations . . . that Cruz is Canadian, a criminal, owned by the banks, etc. . . . or you will lose lots and lots of conservatives.” Levin and others registered no protest when Trump denigrated women, minorities and the disabled. Attacking a favored conservative is evidently a different matter.

But this is Trump’s greatest political talent — exploiting weaknesses like a dentist probing and drilling the most sensitive spot. Trump’s questions about Cruz’s Canadian roots are not primarily about constitutional interpretation. The issue is simpler: Why would voters who support the forced expulsion of 11 million undocumented people want a president born north of the border? Trump’s mention of undisclosed Wall Street contributions highlights the contrast between Cruz’s outsider brand and insider résumé. And Cruz’s seriously Denmark-like proposal for a value-added tax — as Sen. Marco Rubio (Fla.) pointed out in the recent Republican debate — may be disqualifying for many economic conservatives.

This is what Republican presidential candidate Ted Cruz means when he criticizes Donald Trump's "New York values." (Thomas Johnson/The Washington Post)

In a Trump-Cruz battle, I would not bet against Trump. Much of the Republican donor class is convinced that Cruz is the political equivalent of Barry Goldwater, in part because of his very conservative social views. A Trump-Clinton contest, however, is beginning to appear more winnable (particularly as Hillary Clinton appears more awkward and inept). “Donors,” one leading Republican figure told me, “are trying hard to get comfortable with Trump.” And Trump, without doubt, has improved his skills as a candidate.

But here is the problem. Donors, analysts and media are naturally drawn to the horse-race aspect of politics: establishment vs. anti-establishment, insider vs. outsider. But Trump is proposing a massive ideological and moral revision of the Republican Party. Re-created in his image, it would be the anti-immigrant party; the party that blows up the global trading order; the party that undermines the principle of religious liberty; the party that encourages an ethnic basis for American identity and gives strength and momentum to prejudice.

We are already seeing the disturbing normalization of policies and arguments that recently seemed unacceptable, even unsayable. Trump proposes the forced expulsion of 11 million people, or a ban on Muslim immigration, and there are a few days of outrage from responsible Republican leaders. But the proposals still lie on the table, eventually seeming regular and acceptable.

But they are not acceptable. They are not normal. They are extreme, and obscene and immoral. The Republican nominee — for the sake of his party and his conscience — must draw these boundaries clearly.

Ted Cruz is particularly ill-equipped to play this role. He is actually more of a demagogue than an ideologue. So he has changed his views on immigration to compete with Trump — and raised the ante by promising that none of the deported 11 million will ever be allowed back in the country. Instead of demonstrating the humane instincts of his Christian faith — a faith that motivated abolition and the struggle for civil rights — Cruz is presenting the crueler version of a pipe dream.

For Republicans, the only good outcome of Trump vs. Cruz is for both to lose. The future of the party as the carrier of a humane, inclusive conservatism now depends on some viable choice beyond them.

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