The first and most obvious reason is political. The Deferred Action for Childhood Arrivals program, chain migration, sanctuary cities and citizenship for immigrants living in the country illegally will give the left political power. An estimated 70% to 80% of Latin American immigrants will vote Democrat. So, with enough new voters from Latin America alone, the Democrats would essentially be assured the presidency and Congress for decades.
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The second reason for the left's support for virtually unlimited immigration is that one of the most enduring tenets of the left — from Karl Marx to the present-day Democratic Party and left-wing parties in Western Europe — is that the nation-state is an anachronism. [. . .] That's why the left opposes a wall at America's southern border. The wall signifies the affirmation of America as a distinct nation.
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The third reason is the power of feeling good about oneself. It would be difficult to overstate the significance of feeling good about oneself as a primary factor in why people adopt left-wing policies.
Those who support bestowing American citizenship on the children of illegal immigrants — the so-called "Dreamers," based on never-passed proposals in Congress called the DREAM Act — feel very good about themselves. They are the compassionate, the progressive, the enlightened.
This is why German Chancellor Angela Merkel brought a million refugees into Germany, a majority of them Middle East Muslims: She wanted to feel good about herself and Germany — especially in light of Germany's evil history — "Look, world. We Germans really are good people."
Why do Democrats support sanctuary cities, and even sanctuary states? Because, in addition to first two reasons, it enables them to feel good about themselves. In their eyes, they are moral heroes protecting the stranger, the oppressed, the marginalized, the destitute.