As the “tea party’s” outsider challenge to Republican Party orthodoxy grabs headlines, another, quieter revolution is unfolding inside the GOP.

This rebellion has at its heart a truly surprising issue, one that could have long-term consequences for the party: gay and lesbian couples’ freedom to marry.

The latest evidence of this quiet revolution came with the release of the Republicans’ midterm-campaign “Pledge to America.” Though the pledge gives a perfunctory nod to “traditional marriage” (in a single line in a list of things, like “families,” that it supports), explicit opposition to marriage for same-sex couples is conspicuous in its absence.

The document never uses the word “gay” (or “homosexual”) — a stark contrast to past party platforms, which have made opposition to gay equality a centerpiece of their social agenda.

Is this an isolated development? After all, the 1994 “Contract With America” was also focused solely on fiscal issues and government reform. But in 2010, there is compelling evidence that the shift is deep, and possibly lasting.

The GOP, in large part, isn’t displaying its usual anti-gay election-year demagoguery, and not just in the “pledge.”

As recently as 1995, a Republican-controlled Congress was holding hearings investigating “homosexual recruitment” and the “promotion” of homosexuality. During the George W. Bush administration, the party used its fervent opposition to marriage for gay and lesbian couples as a get-out-the-vote strategy, encouraging more than a dozen anti-gay state ballot initiatives geared at driving turnout in the 2004 election and engineering repeated efforts to pass an amendment to the Constitution.

This year is the first election year in recent history in which anti-gay rhetoric has been significantly muted: No state is facing an anti-gay initiative on the ballot, and marriage has not been a focus of the national conservative agenda.

Beyond that, the Republican establishment is stepping up — and coming out. In just the last few months, leading Republican heavyweights have begun to announce their support for the freedom to marry.

Ted Olson, Bush’s solicitor general and a longtime Republican power broker, took the lead, writing articles (“The Conservative Case for Gay Marriage,” in Newsweek) and, with co-counsel David Boies, filing, arguing and winning the first federal court case to uphold gay and lesbian couples’ constitutional right to marry. Cindy McCain and former first lady Laura Bush have both spoken out in support of marriage.

In August, Ken Mehlman, former Republican National Committee chairman and campaign manager for Bush in the 2004 election, revealed that he is gay and supports the freedom to marry.

Mehlman hosted a fundraiser in support of Olson’s lawsuit, with a guest list that would have been unheard of five years ago. It included numerous well-known Republicans such as former New Jersey Gov. Christine Todd Whitman, former McCain strategist Steve Schmidt, former Bush White House communications director Nicolle Wallace and former RNC counsel Benjamin Ginsberg. Bush’s daughter Barbara made an appearance.

Perhaps even more telling, the proudly right-wing GOP Sen. John Cornyn of Texas appeared for the first time at a reception for the Log Cabin Republicans, a leading gay GOP organization. Cornyn had turned down invitations and even contributions from the Log Cabin group in the past.

In September, he weathered significant criticism from the anti-gay Family Research Council. Because Cornyn is chairman of the National Republican Senatorial Committee, and thus tasked with getting Republicans elected to the Senate this fall, every move he makes is watched for ballot-box implications.

What’s driving this insiders’ insurrection? Perhaps a sense that a libertarian-leaning belief in fully extending the freedom to marry to all Americans does not, in fact, clash with a conservative commitment to holding together the social fabric, as marriage entails personal responsibility and social stability.

Or perhaps these GOP leaders are beginning to see an alignment of their rhetoric about individual liberty with public opinion; in the last month, two national polls, by CNN and the Associated Press, showed that a majority of Americans nationwide now support marriage for gay and lesbian couples.

The implications of such a historic shift in the GOP establishment’s stance on marriage should not be underestimated. For Republicans, it means they could become less moored to their socially conservative base and may get back in touch with the cautious but forward-looking American political center that is vital to GOP hopes of cobbling together a governing majority.

For the country, it is evidence that we are inching ever closer to a national consensus that gay and lesbian couples should have the freedom to marry under the law.