The Post’s Chris Cillizza is joined in a Google Hangout by Post SCOTUS reporter Bob Barnes, GLAD’s Mary Bonauto, The Advocate’s Matthew Breen, Heritage Foundation’s Ryan Anderson and SCOTUS blog’s Amy Howe to discuss the cases ahead of the Supreme Court hearings next week. (The Washington Post)

The Supreme Court will hear two landmark cases on gay marriage this week that have the potential to reshape how the country defines one of its most sacred institutions. But, no matter how the high court rules later this year on California’s Proposition 8 and the Defense of Marriage Act, one thing is already clear: The political debate over gay marriage is over.

“There’s no putting this genie back in the bottle,” Florida-based Republican strategist Ana Navarro said Sunday on CNN. “This is now undeniable. The shift is here. We’re not going back.”

Evidence of that reality is everywhere. Dozens of prominent Republicans — led by former Republican National Committee chairman Ken Mehlman — have signed onto a brief to the court urging repeal of Proposition 8, which bans same-sex marriage in the Golden State. Sen. Rob Portman (R-Ohio), a finalist to serve as Mitt Romney’s vice presidential nominee in 2012, announced last week that he was reversing course and would now support the right of gay men and lesbians to marry. (Portman said his decision was influenced by his son, Will, who is gay.)

Anecdotal evidence aside, national polling tells the story in stark terms. In a Washington Post-ABC poll released last week, nearly six in 10 Americans said they support the legalization of gay marriage. That’s the highest level of support ever measured in the Post-ABC survey — and compares with just 41 percent who supported legalization in 2004.

A look inside the numbers makes the case even more strongly. It’s no secret that the issue divides strongly along generational lines, with 80 percent of those ages 18 to 29 supportive of gay marriage, compared with 44 percent of those older than 65. But what’s remarkable is that the generational divide on the question is stronger than the partisan one. In the Post-ABC survey, a slim majority of Republicans and GOP-leaning independents younger than 50 now support gay marriage. Nearly seven in 10 of those age 65 and older oppose it, but that number was more than eight in 10 as recently as 2009.

And, it’s not simply the fact that young people are strongly supportive of allowing gay people to marry that means the political fight on same-sex marriage is over. After all, there is the possibility that as young people age, they might grow less amenable to the idea.

In fact, according to National Opinion Research Center data going back to 1988, each generation has grown more supportive of gay marriage as it has aged. Just 25 percent of baby boomers backed legalizing gay marriage in 2004, but that number had risen to 43 percent in 2012. Ditto Gen X’ers — 37 percent of whom backed gay marriage in 2004 and 53 percent who said the same in 2012.

It’s not just national surveys where the shift is evident. In a Columbus Dispatch poll released Sunday, 54 percent of Ohioans favor repealing a 2004 ballot initiative that established marriage as between a man and a woman in the state’s constitution. (The measure passed with 62 percent of the vote nine years ago.)

All of the above is not to say that the Republican Party will shift its broadly held opposition to gay marriage — at least any time soon. “I can’t imagine that position would ever change,” House Speaker John Boehner (R-Ohio) said last Sunday when asked about the possibility of switching to support gay marriage.

“I still think the Republican Party is going to remain a pro-family, pro-marriage, you know, pro-life party,” Ralph Reed, a leading social conservative strategist, said during an appearance on NBC’s “Meet the Press” last Sunday. “I don’t think that’s going to change.”

To date, it hasn’t; 21 percent of self-identified Republicans supported gay marriage in 2001 while 25 percent supported it 2012, according to Pew Research Center data. What that suggests is that much of the Republican Party remains opposed to same-sex marriage and probably will continue to be so — no matter what the Supreme Court rules later this summer.

But, the trajectory of the data suggests that ambitious Republicans who want to win statewide in swing states or get elected president in 2016 and beyond simply won’t proactively talk about the issue. Outside of Republican primary fights, gay marriage will disappear from the national political dialogue as an issue.

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