However, when it comes to removing the estimated 11 million people living here illegally, costs skyrocket. Ben Gitis and Jacqueline Varas of American Action Forum have calculated that ridding the country of all illegals would take two decades and cost up to $600 billion. “Removing all undocumented immigrants in just two years, as Donald Trump has proposed, would require monumental expansions in U.S. immigration enforcement operations,” they wrote. “Most importantly, under both the 20-year and 2-year time frames, the U.S. economy would shrink by over $1 trillion.”

If Mr. Trump thinks such outlays and trade-offs are worth it, he should show us his cost-benefit analysis. Illegal immigration from Mexico peaked under President Clinton in 2000 and has been in sharp decline since. In many respects, Mr. Trump’s wall would be a solution in search of a problem. In 2014, 230,000 Mexicans were apprehended at the border, down from 1.6 million in 2000. Illegal border crossings today are at their lowest level in 45 years, and net migration from Mexico is zero or negative. According to the Pew Research Center, about 40% of the illegal population entered the country legally and overstayed—another problem that a wall won’t address.