Doing so would seem to show inconsistency, yet there's a strong argument to be made that Clinton's anti-gay-marriage past shouldn't drive decisions to support or oppose her. No one doubts she will be a strong supporter of gay equality if elected president, now that all the political incentives to take that position are aligned. She has advanced gay rights other than marriage at times in her long career. And she has never come across in speeches or interviews as an anti-gay bigot. There is, however, a vocal segment of the left that is invested in likening people who opposed gay marriage to racists who opposed interracial marriage. There is also resentment from gays who feel that the Clintons wronged them in the past.

Andrew Sullivan's perspective is instructive:

She was the second most powerful person in an administration in a critical era for gay rights. And in that era, her husband signed the HIV travel ban into law (it remained on the books for 22 years thereafter), making it the only medical condition ever legislated as a bar to even a tourist entering the US. Clinton also left gay service-members in the lurch, doubling the rate of their discharges from the military, and signed DOMA, the high watermark of anti-gay legislation in American history. Where and when it counted, the Clintons gave critical credibility to the religious right’s jihad against us. And on the day we testified against DOMA in 1996, their Justice Department argued that there were no constitutional problems with DOMA at all (the Supreme Court eventually disagreed). What I’d like to hear her answer is whether she regrets that period and whether she will ever take responsibility for it. But she got pissed when merely asked how calculated her position on this was. Here’s my guess: Unlike Obama, she was personally deeply uncomfortable with this for a long time and politically believed the issue was a Republican wedge issue to torment the Clintons rather than a core civil rights cause. I was editor of TNR for five years of the Clintons, aggressively writing and publishing articles in favor of marriage equality and military service, and saw the Clintons’ irritation with and hostility to gay activists up close. Under my editorship, we were a very early 1991 backer of Clinton – so I sure didn’t start out prejudiced against them. They taught me that skepticism all by themselves, and mainly by lying all the time. So when did she evolve? Maybe in the middle 2000s. Was political calculation as big an influence as genuine personal wrestling? She’s a Clinton. They poll-tested where to go on vacation. Of course it was. But she’s also a human being and probably came around personally as well. She’s not a robot, after all. But I think of her position as the same as the eponymous gay rights organization the Clintons controlled in the 1990s, the Human Rights Campaign. As long as marriage equality hurt the Democrats, they were against it. Now it may even hurt Republicans, they’re for it. So Hillary is for it now. We’ve just got to hope the polling stays strong.

Finally, there is widespread amnesia among Democrats more broadly, who reflexively assume that Hillary Clinton has of course supported gay marriage in her heart of hearts for years. They will experience disillusionment when they discover the truth, especially if they first catch Clinton disingenuously obfuscating. Younger voters in particular are likely to be caught unaware, and then as embarrassed by Clinton's slowness to change her mind as they are about the positions of their own grandparents, who they'd never select to run the federal government. I doubt the conservative Media Research Center had to stand on the street in Washington, D.C., for long to get these reactions from current collegians:

If I were a Clinton primary opponent I'd see that as an exploitable weakness.