The House Judiciary committee advanced the measure on a 7-5 vote after a final public hearing. The bill could be up for a vote on the House floor as early as Wednesday. The Senate passed the measure on a 28-21 vote last Wednesday. Once passed by the House, the bill goes to Gov. Chris Gregoire for her signature, likely next week.
Opponents have promised a referendum challenge at the ballot.
Rep. Laurie Jinkins, D-Tacoma, testified in support of the bill, joined by her partner of 23 years, Laura Wulf, and their 11-year-old son, Wulf.
"We all understand that marriage is not just about contracts and rights and responsibilities," she said. "It's about love and commitment."
Maureen Richardson, the state director for Concerned Women for America, argued that the measure would negatively affect families.
"Marriage is just too important to the culture to be redefined," she said.
Several Republican amendments were rejected, including one that would have added private businesses and individuals, such as bakers and photographers, to the exemption in the measure that doesn't require religious organizations or churches to perform marriages and doesn't subject them to penalties if they don't marry gay or lesbian couples. Another would have added a referendum clause.
Even though lawmakers aren't adding a referendum clause to bill, opponents can still challenge the measure at the ballot with a referendum. Because there is no emergency clause on the bill, the law won't take effect until June 7.
Opponents must turn in 120,577 signatures by June 6 if they want to challenge the proposed law. They are not allowed to start gathering signatures until after Gregoire signs the measure. If opponents fall short in the number of signatures they turn in, gay and lesbian couples would be able to be wed as soon as the signature count is done, likely sometime in June. Otherwise, they would have to wait until the results of a November election.
Washington state has had a domestic partnership law since 2007, and an "everything but marriage" expansion of that law since 2009.
Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the District of Columbia.
Under the measure that passed the Senate Wednesday, the more than 9,300 same-sex couples currently registered in domestic partnerships would have two years to either dissolve their relationship or get married. Domestic partnerships that aren't ended prior to June 30, 2014, would automatically become marriages.
Domestic partnerships would remain for senior couples where at least one partner is 62 years old or older. That provision was included to help seniors who don't remarry out of fear they could lose certain pension or Social Security benefits.
The gay marriage bill is Senate Bill 6239.