MOST AMERICANS SUPPORT MARRIAGE EQUALITY…. It has to frustrate conservatives to know the fight over marriage rights has slipped away from them, and from their perspective, it’s only going to get worse.

Late last year, polls from CNN and the AP found that a majority of Americans support the right of same-sex couples to get legally married. A new Washington Post/ABC News poll, for the first time, found the same thing.

This milestone result caps a dramatic, long-term shift in public attitudes. From a low of 32 percent in a 2004 survey of registered voters, support for gay marriage has grown to 53 percent today. Forty-four percent are opposed, down 18 points from that 2004 survey.

The issue remains divisive; as many adults “strongly” oppose gay marriage as strongly support it, and opposition rises to more than 2-1 among Republicans and conservatives and 3-1 among evangelical white Protestants, a core conservative group. But opposition to gay marriage has weakened in these groups from its levels a few years ago, and support has grown sharply among others — notably, among Catholics, political moderates, people in their 30s and 40s and men.

Of particular interest is the breakdown by age groups. A whopping 68% of Americans under 30 now support marriage equality, but the percentage is nearly as high, 65%, for Americans in their 30s. A majority of folks in their 40s are on board, too.

Older Americans continue to oppose marriage equality, but the trend is still striking — support in this age group has nearly doubled in just five years.

I don’t imagine we’ll ever see 100% unanimity on this question. There’s probably still a tiny percentage of the population that still opposes people of different races or different religions from marrying, too.

But even the most radically anti-gay conservative has to realize that equality is inevitable. As the arc of history continues to bend toward justice, most of the country now believes two consenting adults should be legally permitted to get married if they want to. It’s exceptionally unlikely that trend will ever reverse — civil-rights trajectories simply never move that way. Society becomes less prejudiced, less hateful, and less bigoted over time.

And there’s not much the right can do about it.

To be sure, I don’t really expect conservatives to just throw in the towel — they have too much invested in this — but (a) winning elections by attacking gays is going to be a lot more difficult going forward; and (b) we can safely say marriage equality is only a matter of “when,” not “if.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.