Something very odd and potentially self-defeating is happening to liberalism in the Trump era.

Confronted by the rise of a harder right, the center-left has responded by declaring the intellectual and political equivalent of a public health emergency. Policy positions adopted by their opponents, which liberals of the past would have considered wrong but perfectly legitimate, are now deemed morally unacceptable threats to our form of government — a hazard to the soul of American democracy akin to the danger that an outbreak of a deadly plague would pose to individual American bodies.

Nowhere has this change been clearer or more dramatic than on immigration, and never more so than in reactions to the proposal floated by the White House late last week. In return for providing a permanent path to citizenship for immigrants brought to the country illegally as children, the Trump administration hopes to gain approval for significant cuts to legal immigration.

There are three ways to respond to such a proposal. The first is to make a pragmatic case that cutting legal immigration will harm the economy. The second is to make a moral case that cutting legal immigration will betray America's highest ideals. Both responses implicitly presume that there will be legitimate arguments made on the other side and that those arguments may well prevail in the back and forth of public debate.

But a surprisingly large number of liberals are taking a third, and very different, approach — not claiming that cuts to legal immigration shouldn't be made, but that the very act of proposing and defending them in the first place is morally illegitimate. These liberals appear to believe that immigration restrictionists should be excluded on principle from participating in public debate and discussion about immigration policy in the United States.

This is absurd.

Roughly one-third of the country believes that rates of immigration should be cut. The immigrant share of the population is near historic highs. As the Pew Research Center put it last spring, "a record 43.2 million immigrants [were] living in the U.S. in 2015, making up 13.4 percent of the nation's population. This represents a fourfold increase since 1960, when only 9.7 million immigrants lived in the U.S., accounting for just 5.4 percent of the total U.S. population."

The increase during the intervening five decades was a product of democratically enacted policy. The liberal position amounts to saying that the U.S. should be forbidden from changing this policy, with the country locked into continuing on our current course, no matter what voters think or want.

That is an untenable position in a country that professes to be democratic — yet it is one that growing numbers of liberals are quite eager to adopt.

Unfortunately this isn't the first time they've lapsed into this kind of thinking. It's been building since the middle of the 20th century and has now taken a quantum leap forward under President Trump. We can see it most clearly when liberals are struggling to win a policy debate in the political arena and then turn to the judiciary for help, asking judges or (better) Supreme Court justices to declare opposing policy positions unconstitutional and therefore politically illegitimate. When this happens the losing side must not only accept a temporary political loss but also reconcile itself to a changed reality in which the rules of the political game have been permanently altered to place their position out of bounds from that point forward.

Any political system with judicial review will see such appeals to the refs. But in a healthy political system, such appeals will be rare — and successful appeals, in which policy positions are effectively excommunicated from the democratic political process, will be much rarer.

Judged by that standard, the American political system isn't very healthy right now. Over the past year, federal courts have repeatedly taken stands against Trump administration policies, often on the flimsiest of legal or constitutional grounds. Increasingly, political argument itself is taking the same form, with liberals asserting that Trump Policy X is not just bad for reasons a, b, and c, but that it transgresses some unwritten standard of moral rightness that renders it prima facie unacceptable and illegitimate. Most often the rationale offered for this judgment amounts to the assertion that the policy, or the motive behind it, is racist (or nativist, or xenophobic, or sexist, or homophobic, or transphobic).

And so it has been with the administration's proposal to cut rates of legal immigration, which liberals are lambasting in large part because they are convinced that it (along with its primary author, Stephen Miller) is racist.

The problem is that determining what is and what is not racism is itself partially a political act — which means that liberals can't claim an exclusive right to make such determinations unless they want to be persuasively accused of trying to usurp political power for themselves.

This is abundantly clear from liberal reaction to the administration's proposal to cut legal immigration. During the same decades when the immigrant share of the population has risen to historically high levels, immigration from Mexico has come to dominate. Where once immigrants from a large number of countries were dispersed throughout states across the nation, that is no longer the case.

As a series of useful historical maps, also from the Pew Research Center, make clear, immigration from Mexico began to dominate the Western states around 1980, when the total Mexican-born population was 2.2 million. By 1990, the number had nearly doubled (to 4.3 million), with Mexican immigrants becoming the leading immigrant group in 18 states. Ten years later, the Mexican-born population had more than doubled again, to 9.2 million; now Mexican immigrants were the leading immigrant group in more than half the states. Today the total number of immigrants born in Mexico stands somewhere between 11 and 12 million, with the Mexican-born population leading all other immigrant groups in most of the country outside of the Northeast.

The liberal position appears to be that, even though these trends came about as a result of deliberate changes in immigration policy since 1965, American citizens cannot dislike or wish to alter them in any significant way because that would be racist. Americans may therefore either affirm the status quo or passively accept it, and perhaps be permitted to favor slight adjustments to the mix of considerations that go into the decision regarding who gets approved for work visas and green cards. But actually cutting the number immigrants admitted annually or making changes that could result in a drop in the number of Mexicans relative to those from other countries of origin? That is unacceptable — because, apparently, morality requires that immigration levels remain frozen at their current levels, even if it means that the cultural, linguistic, ethnic, and racial character of the country changes significantly as a result. About such issues, morally acceptable citizens can have no negative opinion.

But of course many millions of Americans do have negative opinions about such trends — and all the finger-wagging and name-calling in the world isn't going to change that. Those millions of Americans are our fellow citizens. They will continue to vote and therefore exercise political power. Can anyone seriously believe that attempting to declare their views beyond the political pale and denying them a seat at the policymaking table will accomplish anything beyond radicalizing them further, potentially sending them outside of the existing party system to do battle with it from an even more extreme position?

Politics at its most fundamental level is a debate about who we are — all of us. The intensifying tendency to set the terms of that debate in such a way that only liberals can take part isn't going end well. Those who end up excluded will claim that their opponents are just trying to win dirty.

And they will be right.