The American Left generally welcomes immigration, but opposes foreign trade. There are exceptions of course, but generally the further left one moves this combination of policy preferences is even starker. Bernie Sanders seems wholly opposed to free trade and yet favors immigration. Indeed, he wants to make citizens of immigrants, even if they have come here illegally.

What explains this divergence? It cannot plausibly be concern for low-wage workers in the United States. It is true that trade, while being generally beneficial, can depress the income of low-wage workers (at least in the short term), because they must compete more with low-skilled workers elsewhere. But the effect of low-skilled immigrants is the same. It puts pressure on the wages of low-skilled Americans.

It can’t be concern for the poor abroad. Certainly, it is also true that immigration to the United States can be a boon to the poor of the world. The move from bad to relatively good governance immediately increases the value of immigrants’ human capital. But trade does the same for those foreigners who can get a higher price for their products and services in a more open market. Indeed, trade has hugely improved the lives of the most wretchedly poor around the globe, helping decrease the number of those who live on less than one dollar a day from 50 percent of the world’s population to twenty percent. And free trade helps many poor foreigners who cannot emigrate to the United States.

Two factors explain the difference in the Left’s preference for immigration over trade. First, the Left likes multicultural societies, and greater immigration helps create and sustain one in the United States. One piece of confirming evidence is that the Left is also less enthusiastic about assimilation. It embraces a variety of policies from bilingualism to ethnic preferences that encourage ethnic consciousness.

Second, the Left gets a political advantage from new low-income immigrants once they become citizens, because low-income voters support the Left more. The multicultural society also helps perpetuate this advantage by making ethnic consciousness a greater determinant of voting, even as immigrants move up the income scale.

I would welcome contrary explanations of the Left’s divergent views of immigration and trade. But for me, the divergence raises questions about the Left’s central claim to be an ideology focused on the improvement of humanity’s lot rather than on the acquisition of raw political power.