The battlelines on immigration have hardened predictably. Left-leaning voters proudly display “refugees welcome” yard signs, while Donald Trump supporters cheer his pledge to implement “largest deportation operation in the history of our country”. Amid such partisan attitudes, it has become heretical to suggest that the Democrats need to be tougher on immigration. But they must. In the long run, progressives have no choice but to acknowledge that huge infusions of migrants stress welfare...