John Derbyshire has a post on immigration that perfectly captures the small-minded nativism that too often underlies opposition to immigration:
A nation has a distinctive culture. The U.S.A., which is much further than the world average from any other consequential country, and has endured several character-forming great national crises, has a culture more distinctive than most. Small boys in 1950s England could pick out an American at 200 yards. Our football (which we love) is nothing like the rest of the world's soccer (which we find extremely boring, and which the rest of the world can keep, far as we're concerned).
Most of the people of a nation are strongly attached to that nation and its culture. (This is called "patriotism." Try the word out a few times. Stress on the firts syllable. It's not that hard to spell.) They like their culture. They don't want to see their culture transformed by uncontrolled mass immigration from places with utterly different cultures.
You may think it would be good for them to have their nation so transformed, but they don't believe you. They like their culture. They're attached to it. They don't want to see it transformed in ways they do not approve, and have never voted for.
Derbyshire is right that America has a culture more distinctive than most. But the rest of this passage gets things completely backwards. What makes America exceptional is not a shared love for American football. We're distinct because we're the first nation explicitly founded on a set of political ideals. Patriotism, as the Founders understood it, was never about blind loyalty to our nation and its political leaders. Rather, the Founders believed that patriotism is about a commitment to the ideals they enunciated in the Declaration of Independence.
Derbyshire thinks that high levels of immigration are a threat to American exceptionalism, but the truth is the exact opposite. A big part of what makes American culture distinctive is our strong work ethic, our disrespect for authority, and our appetite for risk-taking. A big reasons for these traits is the fact that almost all of us are descended from people who valued liberty and opportunity enough to leave everything they knew behind and bear the tremendous costs and risks of crossing an ocean (or more recently, a desert) in search of freedom and opportunity. That steady stream of immigrants has always been an important source of cultural vitality. Whenever America's elites became too complacent, a new crop of freshly minted Americans came along and challenge their dominance.
Derbyshire's counterparts in the 19th Century no doubt warned that American culture would be "transformed by uncontrolled mass immigration from places with utterly different cultures" like Ireland, Italy, and Germany. And they were absolutely right. The details of American culture today are dramatically different from the WASP-y culture that dominated our elite institutions a century ago. It would have been unthinkable a hundred years ago to have a Catholic majority on the Supreme Court, for example.
But we're a better country with a more distinctive culture thanks to the new cultural influences that previous waves of "uncontrolled mass immigration" brought with them. It's important that we teach each new generation of immigrants about the values and ideals that make America distinctive, but there's no reason to think that the current wave of mostly Hispanic immigrants will embrace these ideals any less enthusiastically than past waves did.
The real danger is that if we slam the door shut on new immigrants, our culture will gradually become stagnant and parochial like the countries most of our ancestors fled. In those countries, the defining cultural attributes center around things like what kind of clothes you wear, what kind of food you eat, and what sports you play. We'll know that American culture has truly ceased to be distinctive when we start to define ourselves primarily by our shared love of American football.