As numbers go, "61" languishes in obscurity. But as applied to illegal immigration, 61 is "yuge," as Donald Trump might say. It's the number, in millions, of both legal and illegal immigrants who live in the United States. It's the number, as a percentage of the U.S. population, of Americans concerned about the impact of mass migration on America as we have known it for going on three centuries. The number is further a measure of how Mr. Trump has crossed over from business to politics, to ride the issue to the front of the 2016 Republican presidential campaign.

A study published this week by the Center for Immigration Studies finds there now 61 million immigrants and their American-born minor children living in the United States, 45 million of whom are legal residents. Between 1970 and 2015, the proportion of immigrants to population increased by 353 percent — six times faster than the general U.S. population, which grew by 59 percent.