A U.S. Border Patrol Agent patrols along the U.S. and Mexico border in Naco, Ariz., in 2011. (Joshua Lott/Reuters)

THE ANTI-ILLEGAL immigrant rancor and outright nativism afoot in the Republican primary field give rise to the impression that illegal immigration has soared to unprecedented levels and that the border is no more than a line in the sand, scarcely monitored and easily crossed. The truth diverges wildly from that rhetoric, as a pair of recent studies demonstrate.

Notwithstanding the demagoguery of Donald Trump and some of his GOP rivals, the number of illegal immigrants in this country, which has declined each year since 2008, is now at its lowest level since 2003, and the percentage of undocumented immigrants likewise is at its lowest point since the turn of the century.

A report from the Pew Research Center shows a decline of nearly a million unauthorized immigrants, to 11.3 million, from 2007 to 2014. An even more recent survey, from the Center for Migration Studies, a New York think tank, indicates that the number of illegal immigrants has now fallen to 10.9 million, a precipitous drop driven largely by declining arrivals from Mexico. In fact, according to Pew, for the first time since the 1940s, Mexican migrants have been leaving the United States at a greater rate than they have entered.

Those numbers underscore what demographers have known for several years: that the great wave of Mexican immigration that began in the mid-1960s crested a few years ago and has been tailing off. Some 11.7 million Mexican-born immigrants, roughly half of them undocumented, are now in this country, down from 12.8 million in 2007 . Most of those who have left have done so of their own accord; comparatively few were deported.

That Mr. Trump has leveraged fact-free rhetoric for political advantage is not news. Still, it is noteworthy that so much of the GOP-primary oxygen, at least until the terrorist attacks in Paris, was consumed by alarmist rhetoric about border security, when in fact the border is more tightly patrolled than ever, and apprehensions at the southwestern border, a rough measure of illegal crossings, have been cut by about two-thirds since Sept. 11, 2001.

The dwindling Mexican immigration since the Great Recession reflects a number of factors, including, on the “pull” side, balky economic recovery and tougher border enforcement in the United States and, on the “push” side, improving economic conditions and a falling birthrate in Mexico. The result, according to Pew, is that in the five years ending in 2014, more than 1 million Mexicans (including 100,000 children born in the United States with dual citizenship) returned from the United States to live in Mexico, while 870,000 Mexicans entered the United States, many or most of them illegally.

Republican rhetoric on immigration has not caught up to those numbers, nor to the reality that the U.S. economy, like other Western economies, cannot function without low-wage, low-skill labor, which Mexico has supplied. An estimate 7 million-plus undocumented immigrants, most of them Mexicans, are employed in this country. Mr. Trump’s fantasies of mass deportation notwithstanding, they will not be replaced by native-born Americans. At some point, Republicans will need to grapple with that reality.