As Fox News debate moderators reminded America last week, Donald Trump believes that “the Mexican government is forcing their most unwanted people”—including drug dealers and rapists, who are “doing the raping”—into the United States as illegal immigrants. Trump nonetheless wants to make clear that he has no problem with legal immigration. “Many fabulous people come in from Mexico and our country is better for it,” Trump wrote in a July statement. “But these people are here legally, and are severely hurt by those coming in illegally.” Almost all conservative politicians who believe illegal immigration is a problem make the same distinction: Ted Cruz’s website, for example, says the senator “celebrates legal immigration,” while Marco Rubio used a debate answer to praise the patience of immigrants who wait their turn for a visa. Trump said during the debate that he wants to build a wall on the border but would include “a big beautiful door in that wall” for legal entry.

It wasn’t always this way. Fifty years ago a substantial number of conservative United States senators openly supported severe restrictions on legal immigration, influencing a bill that helped create the modern Mexican “illegal” immigration “crisis” as we know it.

In 1965, President Johnson backed a reform proposal called the Hart-Celler Act (aka the Immigration and Nationality Act), which proposed to eradicate the “national origins” immigration quotas that had been in place since the 1920s. Those quotas prohibited vast non-white portions of the world population (e.g. almost all Africans and Asians) from immigrating to the U.S. while restricting the number of visas allotted to European countries, like Italy and Poland, whose natives were considered ethnically inferior to those from England, Scandinavia, and the like. Under the new system...