Republicans used to have unquestioned, slam-dunk planks. No abortion rights. Fewer gun restrictions. Smaller government. Now, one of the party's former most basic values—marriage defined as between one man and one woman—seems to be slipping into GOP purgatory. Likely presidential contender Sen. Ted Cruz, R-Texas, is hell-bent on stopping that slide.

Same-sex marriage has rapidly become less of a touchstone for Republicans to rally around. Sixty-one percent of the party's voters ages 18-29 say they support the issue, and eight congressional Republicans back it, including possible 2016er Sen. Rob Portman of Ohio. That coalition has the potential to grow: More Republicans running for Congress this year support gay marriage than ever before.

Cruz has never been one to follow popular establishment sentiment. At the Values Voter summit last month, he made a point of sermonizing traditional marriage as a GOP value—and was one of the only prominent Republicans to do so. Sen. Rand Paul, R-Ky., another likely 2016er, declined to mention marriage at all at the summit.

The Supreme Court's decision Monday not to hear the same-sex marriage appeals they were faced with—effectively legalizing it in at least five more states—rocked the country. Responding to this latest development in a shifting tide toward same-sex marriage's legalization, Republican Gov. Scott Walker of Wisconsin acknowledged that the fight in his state was "over." Rather than claim defeat, most Republicans remained silent.

Not Cruz. On Monday, his office released a statement blasting the Supreme Court's decision as "tragic and indefensible."

"By refusing to rule if the States can define marriage, the Supreme Court is abdicating its duty to uphold the Constitution," he said in the statement. "The fact that the Supreme Court Justices, without providing any explanation whatsoever, have permitted lower courts to strike down so many state marriage laws is astonishing."

To fight the Court's tacit approval of gay marriage, he also announced plans to introduce a constitutional amendment blocking the federal government or the courts from striking marriage laws in the states.

For a candidate with his eye on 2016, unapologetically public assertions on gay marriage seem unnecessary. He's made his views on the issue clear before; it's doubtful anyone thought they had changed with this latest development.

But in decisively proclaiming his values on that score, he sets himself apart from potential opponents who have hedged on the issue. Connie Mackey, the president of the Family Research Council's legislative PAC, told National Journal that her group, a harbinger of morality among social conservatives, stands behind Cruz's rhetoric.

"The people in this country are looking for someone who is a bold leader, and who will state his case and defend it," she said. "Ted Cruz and others that would speak up on it will gain the support of the people."

Paul's language on same-sex marriage hasn't been nearly as fiery as Cruz's; in fact, he's said he thinks the GOP's views will "evolve," and that the party can have "people on both sides of the issue." And while Sen. Marco Rubio of Florida said earlier this year he opposes gay marriage, he was also adamant that those views don't make him "antigay," and respects state legislatures right to legalize it if that's what voters want.

If 2012 presidential candidates who ran on social conservativism, such as former Sen. Rick Santorum of Pennsylvania and former Arkansas Gov. Mike Huckabee, join the race, Cruz's uniqueness could disappear. But if they sit it out, Cruz's outspokenness could help him stand out in a primary against such a backdrop. He may be counting on that.

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