Maria Moctezuma, 67, who immigrated to the U.S. from Mexico 25 years ago, reacts as she watches a TV broadcast of the first presidential debate between Donald Trump and Hillary Clinton, in Los Angeles on Sept. 26. (Lucy Nicholson/Reuters)

DONALD TRUMP has telegraphed his intentions, if not always consistently, to radically shift immigration policy and, in so doing, subvert America’s vitality and international standing as a beacon of diversity and tolerance. While he cannot unilaterally undertake every change he proposes, there is plenty he can do, on his own, to overhaul America’s approach to immigrants. His program would undercut the nation’s economic prospects, its values and the vibrancy of its neighborhoods and communities.

A President Trump could slash the number of refugees allowed to enter the country, including from Syria, downgrading President Obama's goal of admitting 110,000 in the fiscal year that started Saturday. Federal law gives presidents the power to bar any "class of aliens" they deem "detrimental to the interests of the United States," so Mr. Trump could as promised ban immigrants from Muslim-majority countries. The effect would be to undercut Washington's standing with allies as well as America's relations with its own Muslim citizens — a blow to America's capacity to fight terrorism.

In the same category of feasible but self-defeating policies, Mr. Trump could revoke work permits and the protection from deportation granted by Mr. Obama to nearly 1.5 million young undocumented immigrants brought to the United States as children by their parents. Just as Mr. Obama has used executive orders to shield those “Dreamers,” Mr. Trump could remove the shield and thereby expel from the country a generation of promising, American-educated young immigrants, most of whom have little or no memory of their birth countries.

As president, Mr. Trump could also unilaterally broaden the categories of undocumented immigrants targeted for accelerated deportation; they would include, as he has pledged, visa overstayers. Together, they amount to more than 5 million people.

True, it would be staggeringly expensive to find, detain, process and remove so many migrants in those categories. Yet considering the pride of place his campaign has given to accelerated deportations, it is likely Mr. Trump could exert his will to a significant extent.

Donald Trump unveiled a 10-part immigration policy plan during a speech in Phoenix on Aug. 31 after meeting with Mexico's president earlier in the day. (Video: The Washington Post)

A study by the American Action Forum, a conservative group, estimates that deporting all illegal immigrants, as Mr. Trump has threatened, would require more than 90,000 federal deportation agents; there are fewer than 5,000. The nation's 34,000 detention beds would have to increase tenfold, and more than 30,000 additional federal lawyers would be needed to process the throngs that would jam immigration courts.

The price of carrying out such a mass expulsion (along with the wall Mr. Trump would build) would not be limited to the estimated budgetary expense ($400 billion to $600 billion), nor even the blow to the economy of depleting the labor force by more than 10 million workers. The most lasting and damaging cost would be to America's prestige globally and to its founding principles. A nation that expels millions of long-standing residents with deep roots in their communities is not a leader; it is a fearful, mean and meek place, heartless and spiritually crabbed. This is not the America envisioned by the Founding Fathers; it is certainly not a home of the brave.

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The clear and present danger of Donald Trump

A President Trump could end the era of American global leadership

A President Trump could wreck progress on global warming

A President Trump could destroy the world economy

How much damage could a President Trump do? We can only begin to imagine.