A woman holds up a signs in support of Deferred Action for Childhood Arrivals, or DACA, during an immigration reform rally at the White House in Washington on Aug. 15. (Jacquelyn Martin/Associated Press)

PRESIDENT TRUMP has outdone himself in dividing America — over banning Muslims, pardoning a racial profiler and spotting “very fine people” among the neo-Nazis and Ku Klux Klansmen in Charlottesville. Now he is reported to be on the brink of dividing families by rescinding deportation protections enjoyed by nearly 800,000 young undocumented immigrants brought to this country as children and raised here to believe in the American Dream.

The president once described the so-called dreamers as “incredible kids” and, saying he’d handle their predicament “with heart,” urged them to “rest easy.” It will be instructive to see if he sticks to that stance in the face of pressure from his nativist attorney general, Jeff Sessions, who has counseled Mr. Trump that he regards the program shielding dreamers from deportation as unconstitutional and indefensible in court.

For Mr. Trump to heed that advice would be in keeping with his own record of pandering to hard-liners in his base by beating up on foreigners. But it would go beyond his usual rhetorical toxicity by upending lives en masse, immiserating huge numbers of people striving to make good.

Dreamers registered with the government starting in 2012 under a program, launched by President Barack Obama, that enabled many of them to attend college, get jobs and driver’s licenses, start businesses, open bank accounts, pay taxes, buy homes and cars, and live ordinary and open lives. All had lived in the United States since at least 2007; none had committed a serious crime. Each signed up for the program, known as Deferred Action for Childhood Arrivals, by trusting the government with their names, addresses and other information marking their emergence from the shadows.

Since Mr. Trump’s inauguration, they have continued registering for two-year permits and renewing them. By doing so, they have hurt no one; to the contrary, they have contributed their energy, ambition and labor to the country and communities where they have spent most of their lives.

That status quo is now challenged by Texas Attorney General Ken Paxton and a group of like-minded conservative state attorneys general who have threatened to challenge the DACA program in federal court unless it is phased out by the administration starting next Tuesday. Like Mr. Sessions, they say it is unconstitutional. In fact, it is a long-standing matter of policy that presidents of both parties have exercised broad discretion in determining whom to deport, given government’s finite resources.

In their letter , the attorneys general insisted that ending DACA wouldn’t require the president to deport dreamers or even rescind the permits already issued. In reality, for every individual who holds a permit, its expiration would mean lost jobs, lost educational opportunities and a lost sense of security. An end to DACA would entail a one-way trip back into the shadows, where immigration agents would be free to track them using addresses and other data they freely submitted when applying for their deferrals.

There is no clamor for ending DACA, which polls suggest enjoys broad public support. Ending the program would be a cruel betrayal and a gratuitous blow against the very people Mr. Trump just months ago described as “incredible.”