A watchdog group is questioning a powerful union’s decision to withhold support from City Council members who cannot be persuaded to agree that union members should be allowed to live outside the city, calling the union’s stance “borderline blackmail.”

The executive director of Citizens Union, Dick Dadey, said yesterday that although it is expected that organizations, including unions, support candidates who support their agenda, “it comes close to crossing a line when an organization ties its support to a particular issue in a kind of quid pro quo stand, as appears to be the case here.”

The executive board of DC 37, the city’s largest public employees union representing 121,000 members, approved a resolution in January “not to support those who don’t support residency,” Executive Director Lillian Roberts said in a statement. A spokeswoman for the union, Donna Silberberg, would not comment on what the union means by “support.”

The union’s new contract, which must be approved by the council, would end a requirement that certain DC 37 members live in New York City, and would allow them to move into one of six nearby counties. Ms. Roberts said it is unfortunate some council members voiced opposition to the provision before consulting the union.

Mr. Dadey said the union is effectively saying in its resolution that it will not deliver votes or campaign contributions to council candidates who disagree with the residency question. The union’s stance was reported by the New York Post yesterday.

“Candidates should not feel forced into taking an issue because of a strong union stand on it,” Mr. Dadey said. “They should not feel like they are being blackmailed if they don’t toe the line.”

Council Member Robert Jackson said many of his colleagues were steaming when they learned of the union’s stance.

“That type of resolution is not positive,” he said. “People felt that they are trying to threaten us.”

Mr. Jackson is opposed to lifting the residency requirement because he said he is afraid it would mean fewer jobs for city residents. He said it is unusual for an organization to make such an ultimatum.

The union “is drawing us a line in the sand,” he said.

A senior attorney for New York Public Interest Research Group, Gene Russianoff, said the union has a constitutional right to decide whether to contribute to any candidate it chooses.

He noted, however, that one reason the union has so much clout is because its members live in the council districts. Thus, if the union does manage to end the residency restriction, it may ultimately weaken its City Hall influence.

“I find it ironic that the effect of their winning on this issue is to have less constituents in the council members’ districts,” he said.

An associate dean and director of the Government Law Center at Albany Law School, Patricia Salkin, said the union’s resolution did not sound illegal. She said it is perfectly acceptable for people to donate to candidates who take positions on issues that a donor supports.