Pittsburgh native Kathryn Lehman was part of the team that wrote the federal Defense of Marriage Act, which the U.S. Supreme Court struck down as unconstitutional Wednesday. But Ms. Lehman was more than glad to bid farewell to the legislation she helped craft.

In 1996, when the law was enacted, she was chief counsel to the U.S. House of Representatives subcommittee on the Constitution. She was also engaged to a man.

Today, Ms. Lehman, 53, is a partner at the Washington, D.C., office of the law firm Holland & Knight. She is also a lobbyist on behalf of Freedom to Marry, an organization that works to advance the cause of same-sex marriage. For the past two years, she has worked to persuade her fellow Republicans to support the gay marriage movement. And she is in a committed relationship with her partner, Julie Conway, with whom she lives in Virginia.

Her political about-face and personal history are no secret in Washington.

But Ms. Lehman is less willing to touch some other subjects.

"Just don't get me into the discussion of Primanti's vs. Wholey's, because it won't get pretty that way," she said.

Ms. Lehman is a third-generation Pittsburgher. Born in Shadyside Hospital, she grew up in Scott, Springdale and Moon. Her immigrant grandparents operated a garage on Centre Avenue in Oakland. Her father, who passed away in 2010, was a choir director at St. George Antiochian Orthodox Cathedral in Oakland and Moon High School, where Ms. Lehman went to high school before leaving Pittsburgh for college and law school.

In 1996, the conservative-led U.S. Congress voted on the Defense of Marriage Act, or DOMA, largely in response to the potential legalization of same-sex marriage by the Supreme Court of Hawaii. DOMA passed through Congress in convincing fashion, with support from both parties.

Ms. Lehman's contributions to DOMA concerned precedents the Hawaii case could set for the rest of the country.

"At the time I am conservative, and I think the idea that one judge making the law for the country just struck me as a not very democratic way to make a decision," she said.

She said she was more involved with writing Section 2 of DOMA -- which permits states that do not have same-sex marriage not to recognize same-sex marriages performed in other states -- than with Section 3, which defines marriage as between one man and one woman.

On Wednesday, the U.S. Supreme Court ruled only Section 3 unconstitutional.

After DOMA was passed, she continued working on Capitol Hill and didn't spend a lot of time thinking about the law. She and her husband split up in 2001. In 2004, same-sex marriage became legal in Massachusetts. Late that year, she started seeing Ms. Conway.

Personal and political circumstances led Ms. Lehman to a change of heart on same-sex marriage. As more states started to legalize same-sex marriage, she observed friends in same-sex relationships who were unable to receive federal benefits because of DOMA.

"It wasn't until you had states adopt same-sex marriage that the impact of Section 3 of DOMA was felt," she said.

In 2009, her friend Ted Olson, a conservative lawyer, took on the Proposition 8 case as a lawyer working on behalf of same-sex couples. It was then that Ms. Lehman completed her political about-face.

"The more I thought about it, the more I was like, 'You know what, [same-sex marriage] makes sense,' " she said.

She believed her new views aligned well with her conservative politics.

"The idea that the [federal] government is protecting the sanctity of marriage is ridiculous to me," she said.

The provision that states be able to define marriage for themselves was also critical.

"I was pleased, obviously, that [DOMA] was struck down, but I was even more pleased that it was struck down because of what was fundamentally wrong with it," she said.

"Who is married had historically been left to the state. All this decision does is restore that," said Anthony Infanti, associate dean for academic affairs and professor of law at the University of Pittsburgh School of Law.

In 2011, Freedom to Marry hired Ms. Lehman to lobby Republicans in Congress. Although 12 states and Washington, D.C. have passed same-sex marriage, she tells them, "the sky hasn't fallen." She also draws on demographic and polling data to argue that this issue is critical to getting younger voters to support the Republican Party.

A handful of Republicans have lent new voices of support to her cause, such as U.S. Sen. Rob Portman of Ohio and U.S. Rep. Ileana Ros-Lehtinen of Florida.

"You've got to give people the space to change their minds," Ms. Lehman said.

She is perhaps the best example of that.

First Published: June 29, 2013, 8:00 a.m.