Gay rights activists march on Washington, October 11, 2009. (AP Photo/Jacquelyn Martin)

Hillary Clinton, who was the first lady of the United States when President Bill Clinton signed the “Defense of Marriage Act,” has recorded a warm and thoughtful endorsement of marriage equality. With the release Monday of her statement, the Democratic Party is completing an evolution on the issue of same-sex marriage. Never again will a serious contender for the party’s presidential nomination—whether Clinton runs in 2016 or not—oppose the right of lesbian couples and gay couples to marry.

But the Democratic Party is not alone in evolving.

Republicans are moving more slowly on the issue. But they are moving. And in many senses this movement provides the most dramatic evidence of the rapid progress being made by proponents of LGBT rights.

When Ohio Senator Rob Portman announced last week that he is now a supporter of marriage equality, he became the latest prominent Republican to abandon the official position of a party that just a few years ago was rigidly—and almost universally—opposed to same-sex marriage.

“I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn’t deny them the opportunity to get married,” said the Ohio senator, who was high on Mitt Romney’s list of vice presidential prospects.

The senator was inspired to abandon his embrace of discrimination at least in part because his son is gay. Other Republicans have been influenced by family connections and friendships. And still others are shifting their stances because they know the Republican Party needs to change.

The Grand Old Party is in a process of transformation that is notable in its scope and character. Less than a decade ago, when states were voting to amend constitutions to formally bar same-sex couples from marrying, leading Republicans were overwhelmingly supportive of discrimination. Now, a growing number of leading Republicans are explicitly rejecting discrimination and taking the position that a new Washington Post/ABC News Poll says 58 percent of all Americans now embrace: “It should be legal for gay and lesbian couples to get married.”

A 2012 presidential contender who was among the top finishers in the New Hampshire primary, former Utah Governor Jon Huntsman, says: “There is nothing conservative about denying other Americans the ability to forge that same relationship with the person they love.”

Huntsman is sometimes accused of harboring moderate tendencies. But no one would dare call former Vice President Dick Cheney anything but a conservative, and Cheney says: “I think people ought to be free to enter into any kind of union they wish.” Cheney has reportedly joined former Republican National Committeeman Ken Mehlman in lobbying Republican legislators around the country to support marriage-equality measures such as the one that was enacted in 2012 in Maryland.

It is true that Cheney’s daughter is an out lesbian, and that Mehlman’s an out gay man. But Americans of all partisan and ideological backgrounds have come to support LGBT rights because of personal connections and experiences. That does not detract from the significance of their commitment to marriage equality. As Cheney says with his regard to his support for same-sex unions: “Freedom means freedom for everyone.”

There are plenty of Republicans, plenty of conservative Republicans, who are coming out for marriage equality on principle. Republicans like former United Nations ambassador John Bolton and former Solicitor General Ted Olson. “We’re talking about an effect upon millions of people and the way they live their everyday life and the way they’re treated in their neighborhood, in their schools, in their jobs,” says Olson. “If you are a conservative, how could you be against a relationship in which people who love one another want to publicly state their vows… and engage in a household in which they are committed to one another and become part of the community and accepted like other people?”

Olson is one of the lawyers aiding the suit that seeks to overturn California’s Proposition 8 ban on same-sex marriage, along with discriminatory laws in other states.

More than eighty prominent conservatives—including former GOP cabinet members and presidential aides, ex-governors and sitting members of Congress—have signed a legal brief arguing that lesbians and gays have a constitutional right to marry.

Of course, there are still throwbacks in the GOP. At the Conservative Political Action Conference last weekend, Florida Senator Marco Rubio drew cheers when he declared to the cardinals, “Just because I believe that states should have the right to define marriage in a traditional way does not make me a bigot.”

Rubio may genuinely reject the argument that his support for discriminatory laws makes him a bigot. But his stance, while it mirrors that of many older Republicans, is not in tune with the young men and women who will shape the party’s future. Among Republicans and Republican-leaning independents under the age of 50, the Washington Post/ABC News Poll found that 52 percent favor marriage equality.

A recognition of that reality underpins at least some of the rethinking within the GOP. “The die is cast on this issue when you look at the percentage of younger voters who support gay marriage,” says Steve Schmidt, who served as a senior adviser to Arizona Senator John McCain’s Republican presidential run.

Schmidt signed the legal brief on behalf of marriage equality.

“I believe Republicans should re-examine the extent to which we are being defined by positions on issues that I don’t believe are among our core values, and that put us at odds with what I expect will become over time, if not a consensus view, then the view of a substantial majority of voters,” Schmidt has been arguing for some time now.

The veteran Republican strategist argues that “denying two consenting adults of the same sex the right to form a lawful union that is protected and respected by the state denies them two of the most basic natural rights affirmed in the preamble of our Declaration of Independence—liberty, and the pursuit of happiness. That, I believe, gives the argument of same sex marriage proponents its moral force.”

That’s an old-school Republican position, one that Schmidt explains is rooted in “the national creed, what Lincoln called the inestimable jewel of American history.” This, says Schmidt, is why he and so many other conservatives have decided to urge fellow Republicans “to respect every human being’s rights to liberty and the pursuit of happiness as much as they cherish their own.”

Schmidt and his fellow Republican supporters of marriage equality are making a big leap away from the right-wing social conservatism that has defined the Republican Party in recent decades. But groups such as the Log Cabin Republicans argue that the move reconnects Republicans with a part of their past. More and more, those who would move the party away from rigid social conservatism argue, Republicans are coming to recognize that the party’s first president was right when he wrote in 1859: “Those who deny freedom to others deserve it not for themselves.”

The Republican Party remains almost completely at odds with its original values on economic issues and a host of other concerns. It is far from the “Party of Lincoln” that it once was. But just as it has been proper to note the Republican devolution away from the best of its historic values, so it is reasonable to note any evidence of an evolution back toward the party’s former self—not merely because of what that suggests about the party’s progress but because of what it says about the broader and better shifting of our national politics.

Republican positioning on voting rights is another story. Read Ari Berman's update on challenges to the Voting Rights Act.