The amicus brief, to be submitted to the high court on Friday, challenges individual states’ prohibitions against same-sex marriage on the grounds of the 14th Amendment, the Reconstruction Era measure drafted to provide “equal protection.”

Joining Baker, organizers of the effort said, are Lieutenant Governor Karyn Polito, former governor William F. Weld, former acting governor Jane Swift, along with Republican National Committeeman Ron Kaufman and Beth Myers, two longtime advisers to 2012 presidential nominee Mitt Romney, himself an opponent of gay marriage.

The state’s Republican establishment is throwing its support behind a US Supreme Court fight against bans on same-sex marriage, led by Governor Charlie Baker, the nation’s only sitting governor to sign on to the GOP brief.


That three of the state’s four living Republican governors have joined a Supreme Court battle in favor of gay marriage underscores how much the issue’s partisan divisiveness, still pervasive in other parts of the country, has dissipated in Massachusetts, the first state to ratify the practice.

Baker said an attorney friend had alerted him to the effort “about a week ago.”

“I read the brief, and it’s pretty consistent with my own views about this issue, and so I told him that I would be willing to sign,” he said in a telephone interview.

Hundreds of other right-leaning figures have signed onto the brief, said lead petitioner Ken Mehlman, the former Republican National Committee chairman who managed President George W. Bush’s 2004 re-election campaign.

The Supreme Court has agreed to review four cases related to gay marriage. The Mehlman brief pertains to a case that originated in 2012 when a same-sex Michigan couple sued the state in federal district court for joint custody of adopted children, then amended the suit to target its gay-marriage ban.

The court has said it will consider whether the 14th Amendment requires states to license same-sex marriages and to recognize marriage rights for a same-sex couple married out of state.


Mehlman said the brief was designed to emphasize that conservatives backing gay marriage are prioritizing freedom, family values, and civil liberties.

“One of the points that I hope people appreciate when they read the brief is that supporting marriage equality is, in fact, the conservative position,” he said Wednesday in a telephone interview.

Noting that the brief differentiated between civil and religious marriages, Baker said, “I certainly viewed it along the same lines as he did.”

“His largest point is that marriage is a good thing, and when it comes to civil marriage there is not a rational basis to say that gay marriage should be treated differently than traditional marriage,” Baker said.

Signers of the brief describe themselves as “social and political conservatives, moderates, and libertarians from diverse backgrounds,” and include libertarian philanthropist and political financier David Koch.

According to brief organizers working with Mehlman, six Republicans in Congress, including Maine Senator Susan Collins, have also lent their names.

Other Massachusetts Republicans who signed on include Richard Tisei, the former Senate minority leader who has run unsuccessfully for lieutenant governor and Congress; Vincent DeVito, an assistant secretary in the Bush Energy Department; and former state representative Patrick Guerriero.

Polito’s name was not initially included in a list of signers provided by brief organizers. Asked about her absence, Baker said he did not know whether she was aware of the effort.


Minutes later, a Baker adviser said Polito had signed on.

During the gubernatorial campaign, a Baker spokesman said that Polito’s views on gay marriage had evolved, from outright opposition to acceptance and support of the institution as settled law.

Democratic Attorney General Maura Healey plans to file a separate brief to the court, and has been soliciting testimony from the public supportive of gay marriage.

While gay-marriage advocacy has become fairly commonplace across the political spectrum in Massachusetts, Baker is something of an outlier among prominent national Republicans.

Early in the 2014 gubernatorial campaign, Baker, as part of a broad effort to soften his image, appeared in a Web video with his gay, married brother, who spoke of the candidate’s “no big deal” reaction when they sat down to discuss the brother’s sexual orientation.

That spot drew national attention because it distinguished Baker from the Republican mainstream, which remains leery of a full embrace of gay rights. The named defendant in the Supreme Court case is Michigan’s Republican governor, Rick Snyder.

Baker has said he has no plans to play a role in national Republican politics, and has stocked his administration with several longtime Democrats. But he also attended a confab of Republican governors in Boca Raton, Fla., shortly after the November election.

On Wednesday, he sounded a note of admonition for others in his party.

“I think that the more we can do as Republicans to make this a nonpartisan issue, the better off we will all be in the long run . . . both as Republicans and as a society,” he said.


A Baker adviser said the governor plans to participate in a national media call Friday intended to highlight support for gay marriage among business leaders and politicians.

Jim O’Sullivan can be reached at jim.osullivan@globe.com. Follow him on Twitter at @JOSreports.