Pennsylvania’s attorney general, Kathleen Kane, said on Thursday that she would not defend the state against a lawsuit to overturn a ban on same-sex marriage.

Ms. Kane, a Democrat, quoting from Pennsylvania’s Constitution, said that unequal treatment based on race, religion and physical disability was no longer tolerated, and “it is now the time here in Pennsylvania to end another wave of discrimination.”

The suit was filed in Federal District Court in Harrisburg on Tuesday with 23 plaintiffs, including 10 gay and lesbian couples, two teenage children of one couple, and a woman whose partner of 29 years had died. It cites a ruling last month by the United States Supreme Court striking down the federal Defense of Marriage Act. It was the first of a wave of planned lawsuits invoking the high court that are aimed at expanding the number of states that allow same-sex marriage, including Virginia and North Carolina.

Attorneys general in Illinois and California have previously declined to defend their states in such suits. In the case of California, the refusal of Jerry Brown, the attorney general at the time, and Gov. Arnold Schwarzenegger led to a second Supreme Court decision last month that cleared the way for gay and lesbian weddings there. When California officials would not defend Proposition 8, a ballot initiative banning same-sex marriage, independent supporters stepped in. The high court ruled that the supporters did not have standing.