Rep. Barney Frank of Massachusetts, one of the country’s most prominent openly gay politicians, said Mayor Bloomberg delivered “a very serious blow” to the cause of legalizing gay marriage.

While Mr. Bloomberg has said he believes same-sex marriage should be legalized, he generated critics on both sides of the gay-marriage debate when he appealed State Supreme Court Justice Doris Ling-Cohan’s February 4 ruling legalizing gay marriage. He said the ruling – which applies only to New York City – was appealed so that the case would come before the state’s highest court, the Court of Appeals, thereby creating uniform marriage law statewide.

Mr. Frank, however, said yesterday in a phone interview with The New York Sun that Mr. Bloomberg’s decision was strategically unwise. He argued that it was politically motivated.

If Mr. Bloomberg and the city were hoping for a Court of Appeals ruling that would settle the gay-marriage question for the entire state, Mr. Frank said, they didn’t need to appeal the Ling-Cohan ruling to obtain it. There are now five cases addressing the same legal question making their way through New York’s court system, and a decision on any of them would settle the city’s dispute as well.

What that appeal does, however, is freeze Justice Ling-Cohan’s decision so that no marriage licenses can be issued to same-sex couples while the case is under review by appellate courts. This, said Mr. Frank, is the real harm done by Mr. Bloomberg’s decision.

The purpose of the appeal, the congressman said, “is to prevent same-sex marriage from going into effect during the interim period” before a higher court rules on the Ling-Cohan decision. Mr. Frank said experience shows that the legalization of gay marriage, even temporarily, is a powerful instrument in convincing a skeptical and reluctant public that same-sex unions are not harmful to society.

In Mr. Frank’s home state of Massachusetts, for example, the Supreme Judicial Court declared in November 2003 that the state’s constitution requires that marriage licenses be issued to same-sex couples; the ruling went into effect in May 2004. A process is under way in the Legislature to amend Massachusetts’s constitution to limit marriage to the union of one man and one woman.

But where public opinion was solidly against same-sex marriage before May, Mr. Frank said, it has now softened, as citizens of Massachusetts have seen that allowing homosexuals to marry has had little impact on their own lives. “The way you deal with these things,” the congressman said, “is to let them go into effect.” Once gay marriage has been implemented for a while, it changes the political debate, Mr. Frank said, “and then people are voting on reality, not fear.”

It was Mr. Bloomberg’s fear of a Republican primary, the congressman said, that led the mayor to deliver the strategic setback to the progay-marriage movement. One of Mr. Bloomberg’s two Republican rivals, a former City Councilmember from Queens, Thomas Ognibene, agreed, saying that Mr. Bloomberg “tried to walk the political fence.” But “everybody saw through his sham,” Mr. Ognibene said, “because he looked completely disingenuous from both perspectives.”

A spokesman for Mr. Bloomberg, Jordan Barowitz, responded: “The mayor makes decisions on the merits, not politics.” The mayor’s other Republican opponent, investment banker Steven Shaw, said he shares the mayor’s views on gay marriage, and said that Mr. Bloomberg “was absolutely right to appeal.”

Mr. Shaw and the city have both pointed to the fiasco surrounding the decision of the mayor of San Francisco, Gavin Newsom, to issue marriage licenses to same-sex couples without any kind of sanction from a court or state legislature. The California State Supreme Court unanimously invalidated the marriages.

Mr. Frank was also critical yesterday of Mr. Newsom, whose actions he says were illegal, “because in America, it is the function of the courts to decide what’s legal.” Public officials, the congressman said, do not make law.

Despite his disagreements with the San Francisco approach to legalizing gay marriage, Mr. Frank said he was “encouraged” by the ruling issued Monday by Justice Richard Kramer of San Francisco County’s Superior Court, a ruling that said California cannot legally ban same-sex marriage.

Meanwhile, attempts by public officials to bring benefits to gay couples were also invalidated in New York yesterday. Local Law 27 – enacted by the City Council to require companies doing business with the city to extend equal benefits to employees’ spouses and domestic partners – was overturned by an appellate court.