Today the House of Representatives reconvenes after spending the summer holding public hearings discussing the alleged dangers of unchecked immigration. The House’s Republican leaders will decide whether to follow the Senate towards moderate immigration reform or whether to follow Representative Tom Tancredo’s hard anti-immigration line and an appeal to immigration’s enemies. The Republican attraction to nativism isn’t new. The party was formed in the 1850s on the ashes of the Whigs, the Free Soilers, and the American Party, a.k.a. the Know-Nothings.The Whigs defined themselves, like the English Whigs, by their opposition to governmental tyranny embodied by Andrew Jackson. The Free Soilers moved towards liberty for all Americans. But the Know-Nothings were defined by opposition to immigration. While I revere the Republican heritage of freedom and limited government, I despise this heritage’s dark doppelganger — the anti-immigrant legacy of the Know-Nothings.

One reason all this should matter to New Yorkers is that an attack on immigration is an attack on cities. Thirty-seven percent of New Yorkers are foreign-born. For centuries, New York City has been the country’s main port of entry. Not coincidentally, Manhattan was at its largest relative to America in 1910, at the height of American immigration. New York’s ethnic neighborhoods still provide a way-station for immigrants combining American urban prosperity with old country comfort. Just as in 1900, it is still a lot easier to keep kosher in Brooklyn than in rural Minnesota. Native Americans have fled to car-based living in the exurbs but less wealthy immigrants value the ability to take the “7” train all the way from the immigrant enclaves in Flushing to Times Square.

Just as cities are good for immigrants, immigrants are good for cities. Agglomeration economies are the essence of urban productivity — more people make cities more productive. But New York City’s population only grew in the 1990s because of immigrants and the city seems to have lost almost 50,000 people since 2000. Unless immigration increases, New York will continue to decline.

Nativism is tied to the view that immigrants pose a threat to our culture. But immigrants are New York’s culture. Since 1655, when the Dutch West India Company welcomed New York’s first Jews, the cultural heritage of every New Yorker has been a heady brew made from scores of societies. German hot dogs, Italian pizza, Dutch place names, Yiddish epithets, Latin music, and English pretension (okay and literature and musicals about felines) are all intrinsic parts of New York’s cultural heritage.

We have been told that the new nativism is justified by a post 9/11 need to secure our borders. Personally, I am much more frightened of nuclear weapons crossing the borders than Mexican immigrants. More significantly, I remain convinced that robust immigration is a bulwark of our national security.American immigrants played a critical role in winning America’s 20th century wars. It wasn’t the worst thing to have our own Germans — Eisenhower and Nimitz — fighting the Axis. Who knows what role the children of Asian immigrants going to Stuyvesant are going to play in the next war? Our 21st century strength depends on our total economic and military might — a more populous America will contribute to that strength.

Our country needs immigration reform that will increase, not decrease, immigration flows. America would have little trouble absorbing a million or more extra immigrants each year, especially with a more sensible immigration policy. We can get tougher on illegal immigration, but we should join that with a dramatic increase in the number of visas.

Our visa policy should be smart. We should favor the young and the skilled who are more likely to contribute to the economy than to tax the safety net. The Australians have long used a points system that favors the educated, and there is no reason not to embrace this type of approach. As an economist, I even like auctioning visas to the pool of suitable immigrants. We can respect national security concerns by favoring those countries that have been America’s recent allies and penalizing those places that fight against us.

I do not have any greater right to American freedom and prosperity than anyone else does, and it pains me that we deny those blessings to the rest of the world. When America closed its borders in 1924, the country lost one of its claims to moral greatness: the promise that anyone could come and be free. Every time we raise our walls, we reinforce our image as Fortress America. I hope the House remembers the promises etched on the Statue of Liberty, “send these, the homeless, tempest-tost to me: I lift my lamp beside the golden door!”

Mr. Glaeser is the Glimp professor of economics at Harvard, director of the Taubman Center for State and Local Government, and a senior fellow at the Manhattan Institute.