Calling it "a day that historians will mark as a milestone for equal rights," Washington governor Christine Gregoire signed into law a state marriage equality bill introduced just weeks ago.

"It is the right step. We have finally said yes to marriage equality," Gregoire said during an exuberant Monday ceremony at the state capitol in Olympia. "The sky-will-fall rhetoric is simply not true. "

The law, which makes Washington the seventh state plus the District of Columbia to offer marriage rights to same-sex couples, is scheduled to go into effect on June 7, unless anti-marriage equality groups gather enough signatures to put the issue on the ballot, which they have vowed to do.

Before that can happen, the bill has to be received by the Washington Secretary of State's office, and a spokeswoman in the office said the measure should be filed by the end of Monday. Opponents are expected to file their request for a referendum as well by day's end.

In a triumphant address before marriage equality supporters, Gregoire thanked the efforts of lawmakers on both sides of the debate, the courage of gay and lesbian youth and the parents who support them, and, with tears in her eyes, her two daughters, who told her that marriage equality was the "defining civil rights issue of their time."

Gregoire was introduced by state senator Ed Murray and representative Jamie Pedersen, two gay lawmakers who were leaders in their respective chambers for the bill's passage. Murray praised Gregoire for her "leadership, passion, and courage."

With the bill signed, antigay groups can now file for a referendum and begin collecting signatures, which would be due by June 6 for ballot consideration. To qualify, they must collect 120,577 signatures.

But Gregoire made a plea to Washington citizens to support the law, saying Monday, "I ask all Washingtonians to look into your hearts and ask yourself, isn't it time for our state ... to support strong families and make Washington state stronger?"

If a referendum were passed, it could face a court challenge, given the Ninth Circuit Court of Appeals' ruling last week that California's Proposition 8 violated the Fourteenth Amendment of the U.S. Constitution when it took the right to marry away from same-sex couples. The two states are similar in that they both have granted broad domestic partnerships to gay and lesbian couples while withholding the title "marriage." They differ in that California had legal marriage equality for several months prior to Proposition 8's passage, whereas Washington will have no such window of time if a referendum qualifies for the ballot.

Gregoire's bill signing was the day's second historic event for marriage equality, with the New Jersey state Senate passing a marriage bill in a 24-16 vote. Gov. Chris Christie has vowed to veto the bill, however. (Read more on the New Jersey vote here.)

Marriage equality advocates and pro-marriage lawmakers praised Gregoire's leadership on the bill throughout the process.

"From her moving remarks endorsing the legislation to her unwavering courage and commitment throughout the legislative process, the Governor has been a key ally in forming the bipartisan coalition that passed this bill," Lacey All, chair of Washington United for Marriage, said in a statement. "We owe her our heartfelt gratitude and sincerest thanks."

Maureen Walsh, a House Republican member who has a gay daughter, was one of several GOP supporters in that bipartisan group. Over the past week, her floor speech in favor of the bill has become an online viral hit.

"When confronted with issues of conscience such as this, I am unable to compromise on what I truly believe is the right thing to do in my heart and in my mind," Walsh said in a Monday statement to constituents. "I signed up for this job, am truly honored to serve, and hope my constituents agree that I must be true to my convictions."

Meanwhile, passage of the Washington bill is likely to prompt a negative response from Republican presidential candidates who reiterated their opposition to marriage equality during appearances at the Conservative Political Action Conference in Washington, D.C., last week. In fact, Rick Santorum will be speaking with marriage equality opponents in Olympia today just a few hours after the governor signed the bill, followed by a campaign rally at the Washington Historical Museum in Tacoma at 7 p.m.