In his too-short life, Joseph P. Overton rose to the level of senior vice president of the Mackinac Center, a conservative think tank in Michigan. He died in 2003, aged 43,

Take gun control: In the U.S. debate, the window encompasses the Second Amendment, so on one side are proponents who think that while there is a right to gun ownership, it should be restricted at least as far as the Supreme Court’s Heller decision will allow, while the greatest opponents in the window believe no restrictions should be applied, and, perhaps just squeezing in, are those who think people could be required to have guns. Who is outside the window? People who think all guns should be confiscated, and people who think they should be able to possess and use an artillery piece with a barrel as long as a telephone pole, for some reason.

There was a little more to Overton’s work than that. He thought that “politically unpopular, unacceptable policies must be changed into politically acceptable policies before they can be enacted into law.”

If we look at where the Overton Window was on immigration to the U.S. just a few years ago, it seems narrow now. Then, we probably would have said we are a nation of immigrants and immigration on balance is a good thing. We should have borders and laws, and they should be enforced within reason. Outside the window would have been separating families and caging minors at the Southern border, along with faith-based bans on immigration; and, at the other end, cutting funding for all immigration enforcement.

In 2015, Bernie Sanders told Vox, “What right-wing people in this country would love is an open-border policy.” Three years later, Trump makes the same charge against Sanders and Democrats. On immigration, we broke the window.

David Frum, the former speechwriter for President George W. Bush, wrote the cover story for the April issue of the The Atlantic magazine, entitled “No Vacancy: If Liberals Won’t Enforce Borders, Fascists Will.” He asks the provocative but necessary and timely question, How much immigration is too much?

“By 2027, the foreign-born proportion of the U.S. population is projected to equal its previous all-time peak, in 1890: 14.8 percent,” Frum says.

“Under present policy, that percentage will keep rising to new records thereafter... Hundreds of millions of people will want to become Americans. Only a relatively small number realistically can. Who should choose which ones do? ...

“If Americans want to shape their own national destiny, rather than have it shaped by others, they have decisions to make now. ... Too little immigration, and you freeze your country out of the modern world. Too much, or the wrong kind, and you over-stress your social-insurance system – and possibly upend your democracy.”

Politicians of all stripes lately have come to treat immigration as a symbol, Frum says, when really it is a system – and one that is not working: “No intentional policy has led the U.S. to accept more low-wage, low-skill laborers and fewer cancer researchers. Yet that is what the United States is doing.”

Partly this is the result of treating immigrants as two distinct groups when there is reason now to see almost all as asylum seekers, Frum contends.

Some of what he is saying is true enough but has become politically impossible to voice, such as that “A smaller immigration intake would dramatically slow the growth in the foreign-born share of the population, better shielding democratic political systems from extremist authoritarian reactions.”

Contained within that statement is the political necessity of finding a way to say it. And by default – for now, at least – the burden of doing it will fall on a Democrat or Democrats who want to lead a way out of this broken window with its jagged glass.