Donald Trump leaves court in Manhattan, where he is serving jury duty, on Aug. 17. (Andrew Burton/Getty Images)

REPUBLICAN PRESIDENTIAL hopeful Donald Trump, who unveiled his immigration platform over the weekend, says America’s illegal immigrants “have to go.” Although the large majority of Americans don’t agree, Mr. Trump is appealing to a more sympathetic audience: the most conservative slice of the Republican primary electorate.

So let’s take Mr. Trump’s plan at face value and examine the impact of deporting millions of undocumented immigrants.

A useful case study is California, whose economy accounts for about 13 percent of U.S. gross domestic product and whose 2.6 million undocumented workers include almost a tenth of the state’s workforce.

For starters, the state’s farms and orchards, where a third to a half of agricultural workers are undocumented, would be crippled. So would their output, comprising more than half the fruits and vegetables consumed in this country. The labor market in construction, where about 14 percent of workers are undocumented, would be severely disrupted. Ditto for hospitality, child care and landscaping.

Mr. Trump says he would keep families together if they include legal and illegal immigrants, but they’d all “have to go.” Does that include the 13 percent of California’s K-12 students who have at least one undocumented parent? How about the U.S.-born children of nearly 4 million unauthorized immigrants nationwide, most of whom have been in the United States for well over a decade?

Republican presidential candidate Donald Trump released a detailed immigration plan over the weekend, and the "great, great wall" is just the beginning. (Gillian Brockell/The Washington Post)

As a quick fix for unemployment, Mr. Trump’s plan is also a non-starter. The share of the labor force occupied by illegal immigrants in California, Nevada, Texas and New Jersey is much greater than the jobless rate in each of those states. Even if every unemployed American in those states took an undocumented worker’s job — wildly unlikely, given that most Americans are unwilling to do the dirty jobs filled by many immigrants — it would still leave hundreds of thousands jobs unfilled.

Despite his nativist rhetoric, Mr. Trump may grasp the staggering economic and social havoc that mass deportation would wreak. Hence his offhand comment, on NBC’s “Meet the Press,” that he’d “bring them back rapidly, the good ones.”

According to the Migration Policy Institute, about 87 percent of the United States’ undocumented immigrants — some 10 million people — have no serious criminal record. If those turn out to qualify as Mr. Trump’s “good ones,” what purpose would be served by deporting them only to “bring them back rapidly”?

What Mr. Trump proposes is nothing less than manufacturing a humanitarian upheaval on a scale rivaling the refugee crisis in Syria. Notwithstanding his cavalier rhetoric, there’s no evidence Americans would tolerate such a mass uprooting of people who have planted deep roots in this nation.

The truth is that Mr. Trump is waging oratorical warfare on a problem whose dimensions have been shrinking for years. The undocumented population declined by nearly a million since it peaked at 12.2 million in 2007. Demographic shifts in Mexico, including a falling birth rate and better educational attainment, are dampening the impetus to leave.

By imposing higher tariffs on Mexican goods, impounding remittances sent by undocumented Mexicans to their families and canceling visas for Mexican businessmen — measures he has threatened as retaliation for Mexico’s supposed complicity in “sending” poor immigrants northward — Mr. Trump would set back Mexico’s economy. The ironic result could be to reignite illegal immigration.