By STEPHANIE BANCHERO and CAROLINE PORTER

Taylor Glascock for The Wall Street Journal

Teacher Jessica Hanzlik says a unionizing push has been a big topic of discussion at her Chicago charter school. She favors the effort.

CHICAGO—Charter schools have spread across the country while generally keeping organized labor out, with operators saying they can manage schools better when their staffs aren't unionized. But labor groups are now making a big push to get a stronger foothold in this educational realm.

Here in Chicago, a branch of the American Federation of Teachers is looking to organize one of the nation's largest nonprofit charter-school groups. Under an agreement last month, the United Neighborhood Organization, which runs 13 charter schools in the city, agreed to provide the union with contact information for its 400 teachers and to let union organizers meet with them on school grounds, even as the charter-school group didn't take a position on whether the teachers should organize.

Backers of charters, which are public schools run by independent groups, say freedom from union contracts enables innovation in areas like staffing and school calendars. Opponents say charters siphon money and students from struggling traditional public schools.

Labor leaders say they want to organize charters because teachers there complain about low pay and poor working conditions, and because unionized teachers can negotiate favorable conditions for students, such as small class sizes. But others say the push has as much to do with unions' declining membership.

"They are betting their futures, to some extent, on getting into charter schools," said Gary Chaison, professor of industrial relations at Clark University in Worcester, Mass.

Officials with the AFT, the nation's second largest teachers union, and the National Education Association, the largest, say they have union drives under way in several districts, including San Diego and Philadelphia. NEA members adopted a resolution last year that "encourages" organizing efforts in charters and directed the national office to share with local chapters "key information" about lessons from previous union drives.

The UNO deal is "very, very, very big," said Randi Weingarten, president of the AFT, who helped negotiate it. "It shows a recognition that the virulent antiunion atmosphere and climate that we've seen in charters may—just may—be changing."

Nina Rees, president and chief executive of the National Alliance for Public Charter Schools, a nonprofit that advocates for charters, said her group wasn't necessarily opposed to unionization as long as contracts don't "bog down" innovation around hiring and firing teachers and lengthening the school day. "My problem is that so many of the traditional union contracts do just that," she said.

Rima Juskys, a special-education teacher at UNO Rogers Park Charter School and a union member in her previous teaching job in a Chicago suburb, said she was "torn" about joining the union. She said UNO expects more of teachers than Chicago public schools do, and that teachers "push and do more for students." But she added that because of tougher working conditions, "there's a lot of worn-down teachers in charter schools, and that's not good."

A 2007-08 survey by the National Center for Education Statistics, part of the Department of Education, found that full-time teachers in traditional public schools earned an average annual base salary of $49,800 and had to work an average 38.1 hours a week to receive base pay, compared with $40,800 and 39.7 hours a week for charter-school teachers.

The charter-school drive comes as teacher unions face headwinds on numerous fronts, including layoffs and curbs to collective bargaining in Wisconsin, Indiana, Michigan and other states. Last year, the NEA had about three million members, while the AFT had about 848,000 full dues-paying members, both declines of about 3% from 2011, according to data from the Department of Labor.

Nationally, about 12% of the approximately 5,000 charter schools in 2010 were unionized, according to the charter-school alliance. That includes charters in states that require most charter teachers be part of districts' collective-bargaining policies. By comparison, more than half of all public-school districts in the U.S., including charters, were subject to collective bargaining in the 2007-08 school year, according to U.S. data.

UNO is run by Juan Rangel, who served as co-chairman of Democratic Mayor Rahm Emanuel's election campaign. Mr. Rangel in the past has blamed the teachers union for impeding education overhauls, and he supported Mr. Emanuel's effort to expand charters—which the Chicago Teachers Union, an AFT affiliate, fiercely opposes.

Mr. Rangel didn't return phone calls. When the union agreement was announced, UNO said it believed it was a "step toward ensuring public education is about collaboration instead of competition and polarization."

The AFT has worked at least three years to unionize charter teachers here. Success with UNO would leave nearly a quarter of Chicago's charter schools unionized—one of the highest penetrations of any city where the law doesn't require charter teachers to be part of collective bargaining, experts say.

Jessica Hanzlik, an eighth-grade teacher at Chicago's UNO Soccer Academy, said teachers at her school have been in deep discussion about unionizing, which she favors. "We see a lot of potential for a union of charter-school teachers to be a powerful mechanism for elevating the profession of teaching," she said.

Write to Stephanie Banchero at stephanie.banchero@wsj.com and Caroline Porter atcaroline.porter@wsj.com