CONCEIVED IN a spasm of xenophobia (Mexican “rapists,” the wall, mass deportations) and weaned on a diet of red-meat nativism (America First!), Donald Trump’s campaign has gloried in demonizing immigrants, particularly the undocumented variety. Which makes it all the more bizarre, on one level, that Donald Trump 2.0 is now hinting at an immigration pivot. “It has to be very firm,” said the suddenly compassionate Republican presidential nominee. “But we want to come up with something fair.”

On another level, what’s notable about Mr. Trump’s putative pivot is its sheer inevitability, given that the policy of mass deportation he previously embraced is politically and economically preposterous on its face. For anyone who aspires to govern, there is just one workable option, which happens to be the same policy Democrats and many Republican leaders have advocated for years: a package of reforms that recognizes the reality of 11 million undocumented residents, most of them employed and with deep, durable roots in American workplaces, families and communities.

Mr. Trump is attempting a neat trick — to marry his newfound consideration for the plight of unauthorized immigrants with the apocalyptic tone he wielded to such incendiary effect in the primaries, and continues to wield in his first general-election TV ad. The ad, which began airing over the weekend in select swing states, portrays immigrants as a dire threat to national security and falsely suggests they collect Social Security benefits. By contrast, the narrator intones, there is Mr. Trump’s America: “Terrorist and dangerous criminals kept out, the border secure, our families safe.” Cue the Border Patrol helicopter.

Just as that menacing message hit the airwaves, Mr. Trump met Saturday with his heretofore unheralded Hispanic advisory council, whose very existence prompts a whiplash of cognitive dissonance. He let it be known that he now seeks a “humane” solution to the problem of 11 million illegal immigrants, which presumably excludes the “deportation force” he previously promised to mobilize to round up undocumented immigrants. Asked about that force, his new campaign manager, Kellyanne Conway, demurred. “To be determined,” she told CNN.

Mr. Trump’s maybe-I-was-just-kidding pivot leaves his intentions opaque and subject to perpetual reinterpretation — a Rorschach test for anyone vaguely enticed by his candidacy. That’s intentional; that’s politics. Yet whatever he may have in mind, certain underlying truths remain immutable.

The first general election television ad from Republican presidential candidate Donald Trump's campaign accuses Democratic rival Hillary Clinton of keeping the "system rigged." Trump released the ad Aug. 19. (Donald Trump)

Those include the fact that at least 8 million unauthorized immigrants are employed, most have been in this country for 15 years or longer, and typically they do jobs — tending crops, washing dishes, mowing lawns — that native-born Americans do not want. In basic economic terms, illegal immigrants meet the labor market’s demand for lower-wage employees, for which there is a shortage of available legal workers.

Even if it were logistically possible to round up and deport millions of such workers, the effect would be not just inhumane, shredding families; it would also be economically catastrophic. That explains the magnetic force that pushes the debate toward reform, even as Mr. Trump’s combustible rhetoric inflames the GOP base against it.