A possible deal with a Los Angeles charter school group has infuriated opponents of the teachers union president, Randi Weingarten, with one opposition leader decrying “the end of the union.” The charter group, known as Green Dot, has been battling its local teachers union over how much to protect teachers.

Ms. Weingarten last month visited Los Angeles and held friendly meetings with each side — and left open the possibility of a partnership with the charter group. “We’d like to build a relationship,” her special representative for high schools, Leo Casey, said.

Leaders of the New York City union’s opposition caucus, the Independent Community of Educators, learned about the trip from a Los Angeles Times editorial, and lashed back in angry blog posts. Green Dot teachers are unionized, but not through the city union, and they lack protections such as traditional tenure or privileges for senior teachers. ICE leaders called Green Dot’s contract anti-teacher.

Ms. Weingarten defended her visit with Green Dot’s founder, Steve Barr, during a meeting of her union’s executive committee, but she failed to satisfy some. “This is the end of the union,” an ICE leader who sits on the executive committee, Jeff Kaufman, said. “She’s going to leave in her wake now a real change in terms of what teachers unions are.”

Mr. Kaufman’s caucus won 10% of the vote in a recent UFT leadership election, but lost all its seats on the executive committee. Mr. Kaufman admitted the blow would dampen the caucus’s power, but vowed to keep up pressure via blog posts.

Some education experts praised Ms. Weingarten’s outreach as a rare display of leadership from a union head, contrasting it with her West Coast counterpart, A.J. Duffy, the president of United Teachers Los Angeles.

The union’s battle with Green Dot escalated last month when teachers at an L.A. high school voted to abandon the public district and join Mr. Barr’s group. United Teachers Los Angeles had fought previous expansion attempts by Green Dot, and a teacher wrote in the Los Angeles Times last week that the union also managed to squash teachers’ push for change. (In an interview, Mr. Duffy denied that any bullying took place.)

Ms. Weingarten’s visit, when she met with Messrs. Barr and Duffy, was an attempt at peacemaking, Mr. Casey said. But he said the trip also continued an ongoing conversation between Ms. Weingarten and Mr. Barr. It had been on Ms. Weingarten’s schedule for two weeks — well before tensions escalated, a union spokesman said.

Ms. Weingarten said she wanted to visit the Green Dot schools, whose union status is unique among charter schools and which boast an 81% graduation rate, in order to see them for herself. After visiting two, she said she was impressed. “They are very teacher-centered,” she said. “It’s obvious, the teacher professionalism and collaboration that is the center of these schools.”

Several sources said Green Dot’s founder has been looking to expand his network into cities beyond Los Angeles. Ms. Weingarten would not say what her next step would be with Green Dot, and Mr. Barr declined to comment for this article. Mr. Casey said the relationship is part of a broader United Federation of Teachers plan to organize what he called the “progressive pole” of the charter school movement, citing groups in Minnesota and Wisconsin.

Many union leaders strongly oppose charter schools, which are privately run but publicly funded. Ms. Weingarten has taken a softer stance, even opening two charter schools of her own.

“She gets that choice is coming to public education, so she’s out in the front, instead of just waiting to get run over by it like some of her colleagues,” Andrew Rotherham, the co-director of an education think tank, Education Sector, said.

During her trip, Ms. Weingarten also met with the philanthropist Eli Broad, who gave Green Dot $10.5 million last year.