For gays and lesbians, the marriage cases being debated at the Supreme Court this week hold the potential for either a landmark expansion or a painful contraction of their civil rights, some narrower changes, or really nothing meaningful at all. But for members of Congress — Republicans, in particular — their political lives will be shaped profoundly by whether the justices go big, go small or essentially stay home on the issue.

The case with the broader constitutional, as well as political reach, being argued Tuesday challenges the California prohibition on same-sex marriage known as Proposition 8. The arguments Wednesday, about the constitutionality of the law denying federal benefits to legally married gay people, hold additional import for Congress as an institution and for every Republican running in 2014. This is especially true for those who voted to enact that law 17 years ago, those who pressed the House to take the legal lead in the current case, and those with statewide or national dreams.

A ruling upholding Proposition 8 would provide the most culturally conservative wing of the GOP a huge shot of momentum for its goal of keeping the party the bulwark against attacks on marriage as an exclusively heterosexual institution. It would also stand to trigger a wave of ballot initiatives on both sides of that issue, complicating the lives of dozens of congressional candidates — especially in California, where advocates of lifting the ban will be counted on to ask voters to do what the court did not.

That would make life particularly difficult for the state’s 15 GOP House members. If they followed the polls and supported gay marriage, they’d run the risk of sustained challenges from fellow Republicans benefiting from the state’s new jungle primary system. If they continued opposing gay marriage, their general elections might be just as problematic.

If the opposite happens and the court strikes down Proposition 8, the political momentum surge would of course be with the politicians supporting marriage equality, a steadily expanding circle in the GOP and an overwhelming majority of Democrats. Whether Republicans follow the lead of their constituents in a wave or a trickle before the 2014 voting would depend on the breadth of the court’s ruling.

A sweeping decision that all same-sex marriage prohibitions are unconstitutional would provide serious political cover for elected Republicans everywhere; they could stop talking about the party’s backing of “traditional family values” and focus instead on their support for the old-fashioned notions of the GOP as the party of individual liberties and equal protection.

Many GOP lawmakers might feel free to do so, but not enough to bring a pre-midterm end to the party’s internal divide, if the court takes the middle course that President Barack Obama’s legal brief suggested, and which many legal experts are now predicting: That California can’t treat gay and straight couples the same in almost every respect without granting marriage licenses. This would have the effect of striking down the bans on gay weddings in the eight other states with everything-but-marriage civil union statutes.

There are also legal rationales for the court to go no further than striking down only the California ban, or to skirt the heart of the matter on procedural grounds. Either of those outcomes would leave GOP congressional candidates in every other state with essentially no rationale for a change in their own stance.

Less wiggle room would be available, especially to current congressional Republicans, under any of the three possible outcomes in the case challenging the heart of the 1996 Defense of Marriage Act, the section precluding legally wed gay people from the spousal benefits afforded to straight married people.

President Bill Clinton signed it — without comment or public witnesses, at just before 1:00 on a Saturday morning — eight weeks before he stood for re-election, positioning himself for the campaign as a social moderate.

Clinton now says that was a mistake, but if the court affirms DOMA it would comfort lawmakers who remain from the overwhelming majorities that enacted it: 14 current Democratic senators, 15 sitting GOP senators, 16 House Democrats and all the GOP House members who remain from that era.

A decisive strike-down, though, would benefit all five senators who votes “yes” in 1996 and are up for re-election next year: Sen. Max Baucus, the only Democrat among them, repudiated that vote nine months ago, so a decision supporting his change of heart might do him some good in Montana, where fewer than 2 in 5 voters back gay marriage.

The Republicans — Sens. Mitch McConnell of Kentucky, Lindsey Graham of South Carolina, James M. Inhofe of Oklahoma and Thad Cochran of Mississippi — could all be counted on to use their unwavering support for DOMA as an antidote to a conservative primary challenge and then a fundraising tool during the fall.

If the court decides not to decide the DOMA case, on the grounds that both plaintiff and the defending executive branch agree the law is unconstitutional, that would be the most important outcome of all for Congress. It would require the justices to agree that the House lacks legal standing to stick up for a law after the president abandons his generally obligatory support for a measure on the books. Such a ruling would be a big-time backfire for Speaker John A. Boehner of Ohio, and could tip the balance of powers against the legislative branch for years to come.

Boehner spent some of his own political capital to insist on pursuing the somewhat novel legal theory of “a direct injury on the House” if the court ruled without its input. If that notion is rebuffed — and we won’t know until June — the procedural and political injuries to two institutions he holds dear will be significant.