First, the population of nuns began to dwindle. Then, droves of parishioners began moving to the suburbs. Now, Catholic schools around the city are facing a new threat to their increasingly tenuous existence: charter schools.

Governor Spitzer’s announcement this month that he is seeking to raise the state cap on the number of charter schools by 150, with 50 of them slated for New York City, had some school choice advocates jumping for joy. Catholic elementary school principals are reacting warily. While happy to see an increase in educational options for parents in the low-income, minority neighborhoods where both charter schools and parochial schools are usually concentrated, some principals are worried that increased competition from charters could exacerbate the enrollment declines in Catholic schools.

The principal of St. Charles Borromeo Parish Elementary School in Harlem, Sister Marianne Poole, attributes the drop in enrollment at her school, to 370 this year from 402 last year, to the opening of charter schools in the surrounding neighborhood. She said many of her school’s best students have moved to charter schools, which often mimic the strict school culture, back-to-basics curriculum, and self-sacrificing teachers frequently featured in Catholic schools.

“It definitely has impacted us,” she said. “If you had an opportunity to get your child into a school modeled on Catholic education, and it’s free, of course you’re going to do it.”

In an article in the spring 2007 issue of Education Next, a publication of the Hoover Institution, a contributing editor, Peter Meyer, describes the rise of charter schools as the latest in a series of hardships facing urban parochial school systems. Other difficulties have included the replacement of older Catholic ethnic enclaves with new Catholic immigrants from the Caribbean and Mexico who are less accustomed to sending their children to parochial schools.

In addition, a dramatic decrease in the number of nuns in the past three decades has meant Catholic schools have had to hire more expensive lay teachers to take their places, forcing the schools to increase tuition to cover the additional labor costs. Nationally, the percentage of female teaching staff in Catholic schools that are members of a religious order decreased to 5% this year from 90% in the 1950s. In the Brooklyn archdiocese, 99 of the 2,130 teachers are nuns, a spokesman, Frank DeRosa, said.

So far, New York’s statewide limit of 100 charter schools has protected parochial schools from the level of competition that has crippled other urban Catholic school systems where there are higher concentrations of charters.

The principal of Sacred Heart School in the Highbridge section of the Bronx, Rachel Suarez, said that while charters haven’t affected her school’s enrollment so far, a lifting of the charter school cap could make a dent.

“Of course if charters do open, parents will have an option,” she said. “Absolutely, there is a concern.”

Both the New York and the Brooklyn dioceses have closed dozens of schools in the past two years because of declining enrollments. This year, Mr. DeRosa said another two schools will close in Brooklyn: St. Charles Borromeo in Brooklyn Heights, which closed this month after a fire in the building, and Presentation in Jamaica, Queens.

Despite the difficulties, Mr. Meyer said the future of Catholic schools isn’t bereft of hope.

“Naturally, the Catholics are feeling somewhat threatened. At the same token, Catholics have a track record of 200 years,” Mr. Meyer said. “The Catholics know that they were the unofficial first charter schools. … They have been — for hundreds of years — educating poor people and educating them very well.”

In several cities, dioceses have reacted to the new challenges by reorganizing schools previously run by individual parishes into clusters in an effort to cut costs and streamline administrative services. The Brooklyn Diocese has begun a restructuring of its schools into regions that include several schools that share staff, such as business administrators and other support services, and the New York Diocese is planning to initiate a similar reorganization of its schools. Some dioceses have also tried marketing schemes, including television commercials.

Mr. Meyer and others also note that voucher programs and tuition tax credits have been helpful in some cases to reduce the competition between parochial and charter schools. A lobbyist for New York faith communities, Michael Tobman, praised Mr. Spitzer for including in his budget a tax credit for private school tuition.

“Calling for a lift in the public charter cap doesn’t come at the expense of also helping families that send their children to private and religious schools — Governor Spitzer has both in his budget,” he said.

The principal of Immaculate Conception School in the South Bronx, Sister Patrice Owens, suggested that Catholic schools should beef up recruiting efforts even while embracing the new schools.

“It does affect our enrollment, but I don’t want to bash them either because I think they are a valid option,” she said. “These are good people who are interested and concerned about the educational system. I don’t think that’s a bad thing.”