Married same-sex couples throughout Washington state and elsewhere would be treated the same as married heterosexuals under a bill to repeal the federal Defense of Marriage Act that was introduced in Congress on Tuesday.

DOMA, as it is known, was passed by Congress in 1996 under then-President Clinton. It defines marriage as between one man and one woman, and bars the federal government from treating same-sex marriages as legal or granting gay couples federal benefits.

The restriction means that even those same-sex couples who married in countries like Canada or who live in states such as Massachusetts, where gay marriage is legal, can’t file joint federal tax returns and aren’t entitled to Social Security or immigration-law benefits.

Hundreds of gay and lesbians couples from throughout Washington state have been married elsewhere.

“None of us expects this will pass in three weeks,” said Jennifer Pizer, an attorney with Lambda Legal, which advocates nationally for the legal rights of the gay community.

“There’s a lot of work that has to be done. We are optimistic and passionate about moving as quickly as is feasible.”

Washington enacted a DOMA statute in 1998, and is among about 20 states with such laws. Federal repeal wouldn’t change the state law, which the state Supreme Court upheld in 2006.

But in 2007 the Washington Legislature established a domestic-partnership registry, passing a law granting same-sex couples some of the same state-offered benefits as married couples. The final expansion of the state’s domestic-partnership law, which the Legislature passed this spring, hasn’t yet taken effect because it is subject to a vote in November as Referendum 71.

With same-sex marriage still a tough sell in the U.S., the House bill introduced Tuesday by Rep. Jerrold Nadler, D-N.Y., and 90 co-sponsors, has a difficult road ahead.

While President Obama has said he’s committed to repealing DOMA, his Justice Department earlier this year urged dismissal of a lawsuit challenging it, noting DOMA remains law until Congress repeals it.

“Washington state is full of gay people who were married in Canada,” Pizer said. “If you are legally married, the federal government (under this bill) would treat you as married — no matter where you were married and where you live.”

Gary Randall, president of Faith & Freedom Network, which is behind the effort to repeal expansion of the state’s domestic-partnership law, said he thinks the gay community is overreaching and predicts the bill will fail.

“There’s this sense of unrestrained enthusiasm to advance the gay agenda,” Randall said. “From all I’ve read, America is firmly committed to marriage as between a man and a woman.”