Though only a few lawmakers participated in the rallies during Tuesday’s oral arguments, more than half the members of Congress had already formalized their views on the same-sex marriage cases before the Supreme Court.

A review of the congressional signatures on three friend-of-the-court briefs revealed an important political narrative underneath the historic story about the future of American society. And that’s the fact almost all the Democrats facing heated re-election races next year have told the court they believe gays and lesbians have a constitutional right to get married. Almost all the Republicans looking at competitive campaigns decided to steer clear of the question.

Who from the Hill joined the briefs and who didn’t, in other words, reinforces the national partisan divide.

Democrats have decided to go all-in behind the surging public sentiment in support of gay marriage, in part by reminding voters their party believes it’s firmly on the right side of history and is eager to persuade the court to hurry up and get there. Republicans remain conflicted on gay rights but understand the electorate increasingly isn’t, so the GOP has decided to get out of the way while expecting the justices are about to take a huge potential political problem off the party’s 2016 worry list.

A brief filed by 167 House Democrats (89 percent of them) and 44 members of the Senate Democratic caucus (all but two) declares the remaining state prohibitions on gay marriage “impose countless burdens and indignities on an identifiable and disfavored class,” inflicting “immeasurable psychological harm” on gay couples and their children while serving “no legitimate governmental objective.”

Every member of the party’s congressional leadership joined the brief. So did the only Senate Democrat currently viewed as vulnerable to defeat next year, Colorado’s Michael Bennet, and five of the seven House members who at this early juncture also seem to be in real re-election trouble: Nebraska’s Brad Ashford, Arizona’s Ann Kirkpatrick, New York’s Sean Patrick Maloney (one of only two House members in a same-sex marriage) and California’s Ami Bera and Raul Ruiz. The two vulnerable Democrats who did not sign, both of whom represent districts Mitt Romney carried for president, are freshman Gwen Graham of Florida and 13-term veteran Colin C. Peterson of Minnesota.

The two Democratic senators who did not sign are West Virginia’s Joe Manchin III, the only senator from his side publicly opposed to gay marriage, and Tom Udall of New Mexico, a former state attorney general who views it as an inappropriate breach of separation of powers for members of Congress to formally take sides in Supreme Court cases.

That’s not the view of Majority Leader Mitch McConnell, who has joined friend-of-the-court pleadings on a handful of Supreme Court cases during his Senate career, most of them concerning campaign finance. He was the only member from the top ranks of the Republican congressional leadership, on either side of the Capitol, among the six GOP senators and 51 House members who signed a brief urging the court to maintain the status quo. (The most consequential House signatory was Judiciary Chairman Robert W. Goodlatte of Virginia.)

“This court favors incremental change over sweeping and dramatic change in addressing novel constitutional claims. The relative novelty of same-sex marriage weighs against the mandatory redefinition of marriage,” Republicans wrote, and so the court should permit states to continue using the traditional definition of marriage as the union of a man and a woman.

The justices are considering challenges to bans on same-sex marriage from McConnell’s Kentucky as well as Tennessee and Ohio. None of the four other GOP senators from those states bought the brief’s line of argument: presidential candidate Rand Paul, Lamar Alexander, Bob Corker or Rob Portman, who in 2013 became the first Senate Republican to declare support for gay marriage. (Michigan’s ban also is part of the case; its senators are Democrats.)

The only presidential aspirant from the Senate who signed was Texan Ted Cruz, who is hoping to win the GOP nomination by engineering a surge of support from cultural conservatives.

None of the eight Republican senators preparing for competitive re-election races decided it was a good idea to put their names on the anti-gay-marriage legal brief. But one of them, Mark S. Kirk of Illinois, did the opposite — signing a brief declaring that affording marriage rights to same-sex couples advances “conservative values” and declaring that banning gay marriage is inconsistent with the “properly limited role of government.” Kirk was on the scene outside the court Tuesday as the arguments were heard.

That pro-marriage brief also was signed by more than 275 other Republicans, including Sen. Susan Collins of Maine, Gov. Charlie Baker of Massachusetts, 17 former members of Congress, billionaire David Koch, prominent Hill staff veterans including former House Clerk Jeff Trandahl, several senior Romney campaign staffers and an array of other GOP luminaries ranging from retired Gen. Stanley McChrystal to late 1980s era Roll Call editor James K. Glassman.

Three sitting House members joined them: Ileana Ros-Lehtinen of Florida, Richard Hanna of New York and Robert J. Dold of Illinois — who stands out because, like Kirk, he’s looking at a tossup race for-re-election next year.

Besides Dold, there are 20 House Republicans currently considered something other than a safe bet for re-election, according to the initial race ratings by The Rothenberg & Gonzales Political Report/Roll Call. But 19 of them decided to keep their powder dry altogether on the gay marriage case.

The only one willing to campaign in a tough race as an opponent of marriage equality is Tim Walberg, a Christian pastor before coming to Congress. His race for a fifth term in southeastern Michigan looks to become an unusually open waging of the culture wars. The likely Democratic challenger is state Rep. Gretchen Driskell. In 2011 when she was the new mayor of Saline, outside Ann Arbor, Driskell declared June as Lesbian, Gay, Bisexual and Transgender Pride Month in her small city.

Related:

Court, Not Congress Could Mark Civil Rights Landmark

Questions for Portman on His Gay-Marriage Conversion

Latest Partisan Divide: Religion and Politics Should Mix

Rothenberg & Gonzales Political Report/Roll Call Race Ratings

The 114th: CQ Roll Call’s Guide to the New Congress

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