As the Senate debate on immigration reform moves into its second week, the solons will face some of their most serious tests yet. The judiciary committee last week voted out a bill that would hire tens of thousands of new border agents while also increasing opportunities for new immigrants to enter legally and offering those who are already here illegally a chance to pay a fine to move out of the shadows and on with their lives. Opponents are expected to try to gut that proposal this week, offering amendments either to strip out the guest worker program at the heart of the committee’s bill or to offer a completely different enforcement-only proposal.

New York’s senators have expressed support for real reform – Senator Schumer voted in favor of the committee’s bill – but because they have chimed in on the debate relatively recently, their constituents will be keeping an especially close eye on Mr. Schumer and Senator Clinton to make sure they don’t get weak-kneed. The pair owe it to their constituents to stand firmly behind the judiciary committee’s bill. New York is too dependent on immigration to countenance anti-immigrant measures masquerading as reforms.

New York City as we know it today would not exist without immigrants, and not just the immigrants of the 19th and early 20th centuries who brought us St. Patrick’s Day, kosher delis, Little Italy, and kielbasa. According to a study prepared by New York’s Department of City Planning, about 36% of the city’s population today was born overseas; without immigrants, New York would be home to 2.9 million fewer people. Brooklyn alone would be nearly 1 million inhabitants short.

The population gain extends to the second generation. Immigrants tend to have higher birth rates than native-born New Yorkers. Foreign-born mothers give birth to more than half the babies born in the city. More than 60% of babies born in the city have at least one immigrant parent. Immigrants and their American born children comprise about 55% of the city’s population. The combination of high immigration rates and high immigrant fecundity mean that immigration has spared New York from the scourge of many other large American cities – population decline.

While other large and old urban areas have been steadily depopulated by flight to the suburbs, large immigration flows into New York have offset the effects of such departures by people who have lived here longer. Between the 1990 and 2000 censuses, New York City experienced a net out-migration of 136,000. Although 475,000 people left the city for other parts of America, 339,000 immigrants arrived from foreign countries. As the planning department’s report puts it, “The flow of immigrants to New York mitigated catastrophic population losses in the 1970s, stabilized the city’s population in the 1980s, helped the city reach a new population peak in 2000, and continues to play a crucial role in the city’s population growth.”

Population growth is not merely a matter of ensuring New York beats Los Angeles for the title of America’s largest city. It’s a matter of economic health. In 2000, 43% of New Yorkers in the labor force were immigrants. Between the ages of 25 and 54, between 40% and 50% of those in the labor force were immigrants. Immigrants are building the city (58% of construction workers), feeding it and turning down the sheets for its guests (54% of workers in the hospitality and other services sector), and keeping it healthy (41% in the educational, health, and social services sector).

Immigrants and their children are part of the fabric of New York City life and have been for generations. Consider the long list of important people both living and dead who either settled in the city as immigrants themselves or were born of immigrants in New York before moving elsewhere: Alexander Hamilton, Marcus Goldman (founder of Goldman Sachs), Elie Wiesel, Yoko Ono, Lou Gehrig, Irving Kristol, Henry Kissinger, Tina Brown, George Soros, Vince Lombardi, Jagdish Bhagwati, Frank McCourt, Fiorello La Guardia, Isaac Asimov, Mikhail Baryshnikov, George and Ira Gershwin, B.C. Forbes (founder of the publishing dynasty), Patrick Ewing, Felix Frankfurter, Dr. Ruth Westheimer, Gisele Bundchen, and the list goes on. New York’s acceptance of immigrants has enriched the city and led it to exercise enormous sway throughout American society.

About 80% of the immigrants in the city come to America legally. Because those immigrants are so important to the city, New Yorkers have a stake in provisions of the Senate bill that would expand opportunities for legal immigration. Backlogs for green cards in the city currently stretch for years, and as increasing numbers of people are turned away for legal entry, New York has already started seeing an increase in the number of illegal immigrants. The city needs Washington to act now to prevent that trend from expanding, by opening the doors to more legal immigration.

The time is ripe for real improvements to the immigration laws. An Associated Press poll released over the weekend found that 56% of Americans would support the kind of earned legalization provision in the Senate bill that would allow illegal immigrants to pay a fine and back taxes in order to get on track for legal status. Two-thirds doubted that a wall along the Mexican border would help. Among Republicans – members of the party leading the anti-immigrant fight in the House – support for earned legalization is 52%.

But pushing this bill through Congress will mean persuading several skeptical senators and a lot of anti-immigrant members of the House. It will be a hard fight that will require strong support on the Hill. Mr. Schumer and Mrs. Clinton have said they would support a bill like the judiciary committee proposal. They have seen on their trips home the importance of immigration for their constituents. This week is the time for them to step up to the plate to make sure New Yorkers – and Americans – get the immigration reform we need.