In 2005, a left-leaning blogger wrote, "Illegal immigration wreaks havoc economically, socially, and culturally; makes a mockery of the rule of law; and is disgraceful just on basic fairness grounds alone." In 2006, a liberal columnist wrote that "immigration reduces the wages of domestic workers who compete with immigrants" and that "the fiscal burden of low-wage immigrants is also pretty clear." His conclusion: "We'll need to reduce the inflow of low-skill immigrants." That same year, a Democratic senator wrote, "When I see Mexican flags waved at proimmigration demonstrations, I sometimes feel a flush of patriotic resentment. When I'm forced to use a translator to communicate with the guy fixing my car, I feel a certain frustration."

The blogger was Glenn Greenwald. The columnist was Paul Krugman. The senator was Barack Obama.

Prominent liberals didn't oppose immigration a decade ago. Most acknowledged its benefits to America's economy and culture. They supported a path to citizenship for the undocumented. Still, they routinely asserted that low-skilled immigrants depressed the wages of low-skilled American workers and strained America's welfare state. And they were far more likely than liberals today are to acknowledge that, as Krugman put it, "immigration is an intensely painful topic ... because it places basic principles in conflict."