Dick Cheney’s support for gay marriage (driven in part by his own daughter’s sexual orientation) was something of a paradigm shift for the Republican Party.
Rob Portman’s sudden declaration that he now backs same-sex marriage is going to further accelerate the move towards wide-spread acceptance.
Portman is extremely well liked within the party. The op-ed he wrote explaining his new position (and its personal nature) reflects his usual humanity, humility, careful reasoning, and determination. Very few conservatives denounced Cheney for his view on the matter. I doubt almost anyone from the right will criticize Portman either.
For several years now there has been speculation in political quarters about when a Republican with presidential aspirations might risk public support of gay marriage. After today, the answer is pretty clear: 2016.
And it might not be seen as that big a risk.