Portman is a pillar of the political establishment -- he held two different Cabinet posts in George W. Bush's Administration -- served in the House, and was widely mentioned as a potential vice presidential pick. That someone with his profile -- his own personal circumstances notwithstanding -- would reverse positions in such a high-profile way tells you much about how the politics of the issue are shifting.
Portman's announcement comes on the heels of more than 100 big-name GOPers signing a brief to the Supreme Court in favor of gay marriage -- an effort organized by former Republican National Committee Chairman Ken Mehlman, who is gay. (The Supreme Court is expected to hear a challenge to California's gay marriage law later this month.)
That's not to say that allowing gay people to get married will suddenly become the default position of the Republican party. It won't. In fact, polling suggests that while the country writ large is growing more and more tolerant of gay marriage, Republican voters aren't.
And simply because Portman and some other high-profile figures -- including President Obama -- have changed their minds on the issue doesn't mean that the party as a whole will follow their lead. Portman has always been known as a fiscal conservative only loosely concerned with social issues like marriage and abortion, and so his announcement isn't likely to have major ripple effects among social conservatives broadly.
But the gay marriage issue does appear to be on a trajectory similar to the gun issue for Democrats from the mid 1990s all the way up to the recent spate of mass shootings that culminated in Newtown, Conn., late last year.
In the mid 1990s, Democratic leaders -- up to and including Bill Clinton -- simply stopped talking about guns and gun control. That didn't mean that a sizable portion of the Democratic base stopped caring about the issue; they did and still wanted more and stricter gun control measures.
But the politicians in the party recognized that the broader public was simply not on their side on the issue and, in order to win elections, they needed to de-emphasize it. They didn't abandon their past positions, they just stopped talking about it. Entirely.
A look at gay marriage polling broken down by party identification suggests something similar is likely to happen on this issue for Republicans.
The fact that 54 percent of Republicans strongly oppose gay marriage means that the party won't be changing its official position on the issue any time soon. And we would bet that most of the frontrunners for the 2016 presidential nomination won't follow Portman's lead in coming out in support of gay marriage.
But, what almost certainly will happen is that the likes of Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Wisconsin Rep. Paul Ryan will talk about gay marriage less -- or, at the very least, in a different tone -- than they would have ten years ago. Talking about gay marriage -- at least expressing opposition to it -- will be akin to supporting stricter gun control as a Democrat: You do it only when speaking to your party's base.
That shift is a reflection of a political reality that's best understood not by slicing the country along party lines but rather along generational ones.
The simple fact is that young people -- defined as those under 30 (so depressing for a 37-year old Fix) -- are broadly supportive of gay marriage. In a recent Washington Post-Kaiser poll, seven in 10 respondents between 18 and 29 said they thought it should be legal for gay couples to marry. Among Republicans aged 18 to 29, 50 percent thought it should be legal, while 46 percent said it should be illegal.
Politically speaking, the writing is on the wall when it comes to gay marriage. It will -- and is -- becoming an issue that remains very important to a segment of the Republican base and one that in certain situations can animate that base to action. But it simply is not an issue that Republican politicians aspiring to national office will talk much about in 2016 and beyond.