One legal expert said Kane's stance put her in a gray area.
"It's not her job to substitute her judgment [on the law's constitutionality] for that of the courts," said Bruce Ledewitz, a Duquesne University law professor. "And though I don't like the law, that is our law. And she is not serving the people of Pennsylvania by not defending it."
State law says the attorney general must defend Pennsylvania laws, but can ask lawyers for the governor's office or executive-branch agencies to step in if that is in the state's best interest.
Gov. Corbett, a Republican who opposes same-sex marriage, had no immediate comment Thursday. Kane said she did not speak with him before announcing that she would not defend the ban against a lawsuit filed Tuesday by the American Civil Liberties Union.
Corbett's general counsel, James Schultz, said the administration was surprised that Kane, "contrary to her constitutional duty," had decided "not to defend a Pennsylvania statute lawfully enacted by the General Assembly."
The administration was awaiting formal notification of Kane's decision and an accompanying legal justification, he said.
House Republicans sent Kane a letter quoting James Madison on the dangers of tyranny and essentially saying: Do your job.
"There are any number of Pennsylvania statutes with which we may personally disagree," said the letter. "Nevertheless, we do not ignore them to suit our political preference."
In Philadelphia, Kane described a continuum of civil rights laws that allowed women to vote, struck down segregation, and eliminated bans on interracial marriage.
"I cannot ethically defend the constitutionality of Pennsylvania's version of DOMA," she said, referring to the Defense of Marriage Act. "I believe it to be wholly unconstitutional."
The decision by the state's top law enforcement official may ultimately mean little in the court defense of the 1996 law. But it was heralded by activists and gay couples as a victory in the court of public opinion.
"It's absolutely huge and emotionally powerful," said Molly Tack-Hooper, one of the lawyers in the ACLU suit.
That suit, filed in federal court in Harrisburg by 10 gay couples and others who say their rights have been trampled, came less than two weeks after the U.S. Supreme Court struck down a federal law that defined marriage as being between a man and a woman, and amid national polls that show growing support for gay unions.
Kane spoke on a crowded second-floor area of the Constitution Center, faced by television cameras, supporters, curious tourists, and a Boy Scout troop that was touring the center.
"I'm speechless. That was great," said Dara Raspberry, there with her spouse, Helena Miller, and their 6-week-old baby, Zivah. The Philadelphia couple, married in Connecticut, is among the plaintiffs in the ACLU suit.
State Rep. Brian Sims (D., Phila.), who last fall became the first openly gay candidate to win a seat in the legislature, posted on Twitter Thursday morning: "Attorney General Kathleen Kane, if you were a man I'd marry you!"
He later wrote, "Marriage equality will happen in Pennsylvania, the only question remaining is 'when?' "
But State Rep. Daryl Metcalfe (R., Butler), one of the most vocal supporters of the ban, called Kane's decision "a political maneuver" and "a dereliction of duty."
"She has a duty to defend the laws of the state, whether she agrees with them or not," said Metcalfe, chairman of the House State Government Committee.
Metcalfe said he supported the ban because "throughout history, marriage has always been between one man and one woman, through multitudes of religions and multitudes of cultures."
He said that if Kane "doesn't want to do her job, then she should resign and allow Gov. Corbett to appoint someone who understands the responsibilities and duties of that office."
Montgomery County pastor Bill Devlin said Thursday night that he and other clergy across the state would soon press Kane with the question: What other laws will you not defend?
"She swore an oath to uphold the constitution of Pennsylvania, which includes supporting laws she may personally disagree with," said Devlin, cochair of a group called Right to Worship. "How dare she not stand up for the citizens of the Commonwealth who in a representative, democratic fashion have said we believe marriage is between a man and a woman?"
Kane is named as a defendant in the ACLU suit, as is Corbett. With her withdrawal, the general counsel's office would be in charge of defending the state, Kane said.
Philadelphia lawyer Mark A. Aronchick, whose firm is working with ACLU, called her stance "one of the most principled and professional actions I have ever seen from a public official." To hear the state's chief law enforcement officer basically agree with the plaintiffs was "an earthquake moment," he said.
The suit seeks to prevent state officials from stopping gay couples from marrying. Lawyers in the case believe it could reach the U.S. Supreme Court, along with similar cases from elsewhere.
Thirteen states and the District of Columbia have legalized gay marriage. New Jersey legislators approved a same-sex marriage bill that Gov. Christie vetoed, saying the issue should be decided by referendum.
Kane's announcement made national news.
"We applaud Attorney General Kane for doing the right thing, standing up for equal justice under the law and fighting legally sanctioned discrimination in her state," said Matt Mittenthal, spokesman for New York state Attorney General Eric Schneiderman.
On Thursday, Kane quoted Robert F. Kennedy, who in a famous 1966 speech in South Africa said that when a person stands up for an ideal, "he sends forth a tiny ripple of hope, and those ripples build a current which can sweep down the mightiest walls of oppression and resistance."
That failed to resonate with Republicans.
"She is blatantly politicizing the highest law enforcement office in our commonwealth," Gleason said, "at the expense of a core responsibility of the Attorney General's Office."
Contact Jeff Gammage at 215-854-4906, firstname.lastname@example.org, or follow on Twitter @JeffGammage.