Juan Campis was sweating in the 90-plus degree heat as he whipped a white towel across a gleaming black Chevy TrailBlazer at a carwash here—one of six in the city that was unionized in recent months with help from two nonprofit community groups.

“They’re the ones that kept us all together and showed us the steps we needed to take,” Mr. Campis, 20 years old, said of the community groups. Workers probably wouldn’t have joined the union without daily contact from the two groups, he said.

The community groups, called worker centers, are often backed by unions. But they aren’t considered “labor organizations” by law because they don’t have continuing bargaining relationships with employers. That gives them more freedom in their use of picketing and other tactics than unions, which are constrained by national labor laws.

The new approach is sparking a backlash from some businesses, who call it an end-run around labor laws that can be used to help unionize new groups of workers.

The Center for Union Facts, which opposes organized labor and gets much of its funding from corporations, said it is launching an advertising campaign criticizing ties between unions and worker centers.

“It’s a more potent strategy than unions have used in the past,” said J. Justin Wilson, managing director of the Center for Union Facts. Worker centers are “winning hearts and minds with positive things like language classes, while worker centers create strife and conflict within a company.”

John Raudabaugh, a former Republican member of the National Labor Relations Board, said he believes most worker centers have limited their activities to community activities permitted under the law. But he expects more legal challenges to decide whether the groups are crossing a legal line. “The question is, have those organizations had conversations with the employer or is it just one-way pressure?” he said.

As union membership has fallen to 6.6% of the private sector, organized labor sees the worker centers as a new way to grow its ranks while mobilizing workers of all stripes.

Workers who are first organized by worker centers can be later organized by unions. That is what happened with nearly 200 carwash workers in Los Angeles and New York, organized by the Retail, Wholesale and Department Store Union and the United Steelworkers. The steelworkers union also is trying organize several hundred pizza factory workers in Wisconsin who joined a worker center.

“It’s the future of organized labor,” said Stefan Marculewicz, an attorney who represents companies in labor disputes. “A lot of companies are looking at this and saying ‘When am I next?’ “

The New York Taxi Workers Alliance claims 17,000 members, while the National Domestic Workers Alliance has 10,000 members who work as housekeepers, nannies and other caregivers. The taxi workers group is one of several worker centers that have affiliated with the AFL-CIO and in turn pay a membership fee to the federation.

Twenty years ago, there were only a handful of worker centers in the U.S. Many of those were formed in southern states—where there were few unions—to work with textile workers.

Today, there are more than 200, some with very specific missions, such as the Koreatown Immigrant Workers in Los Angeles and the Coalition of Immokalee Workers, which works with immigrant workers who harvest tomatoes in Florida.

Some worker centers target immigrants, organizing taxi drivers, day laborers and domestic workers who can’t join unions because they are independent contractors. Others lobby politicians and help workers with complaints like unpaid wages.

Last year, the Service Employees International Union gave $2.5 million to New York Communities for Change, more than the group’s total income of $2.1 million in 2011, according to its latest available tax filing. The group, based in Brooklyn, was formed in 2010 by leaders of the former New York chapter of the community organizing group Acorn, after many of that group’s affiliates disbanded.

Earlier this year, the New York Communities for Change kicked off a series of one-day strikes involving more than 2,000 fast-food workers at dozens of chains in more than half a dozen cities. The workers are asking for $15 an hour and “a fair process” to join a union—they don’t specify which one.

The carwash campaign in New York illustrates how unions and worker centers work together.

The plan to organize the city’s 200 carwashes, which employ about 5,000 workers, many from Mexico, Guatemala and El Salvador, came out of a 2011 meeting between leaders of NYCC and the Retail, Wholesale and Department Store Union at the union’s Manhattan offices in 2011, according to union President Stuart Appelbaum. Also involved was another larger community group, Make the Road New York, which has 13,000 members and charges $120 in annual dues.

Tasks were divided up: The community groups would build community support to back an organizing campaign. The union would file for elections with the National Labor Relations Board.

“These are people who felt that they were living in the shadows and powerless and that nobody cared,” said Mr. Appelbaum.

He said unions recognize they need to create broader coalitions with groups that provide credibility within communities to organize workers.

In the two years since the initial meeting, the union has given about $107,000 to NYCC and about $94,000 to Make the Road New York, according to government filings. Mr. Appelbaum said the union supports the groups for other advocacy work, such as advocating raising the minimum wage and a campaign for a paid sick-leave law in New York.

“I don’t think you can say that we’re funneling money to them for the carwash campaign,” he says. “We contribute to all sorts of groups.”

Jonathan Westin, executive director of New York Communities for Change, said there will be growing cooperation between unions and community groups. “If we’re ever going to maintain decent wage levels, it can’t just be insular union fights.”

The union has filed for six elections at New York carwashes and won them all. Under two union contracts now in place, workers receive $7.53 an hour including tips in the first year of the contract, up from $7.25 an hour. They will earn $9.18 an hour in the third and final year of the contract. They also receive five paid sick days for the first time.

Both Luis Rosales, 22, who arrived from Mexico eight years ago, and his father work at a Queens carwash. At first, he said, they were afraid of retaliation if they joined a union. “It took months to unify as a group to say we want to fight the fight,” he said through a translator. He and his father voted to join the union in May.

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