Even as a fresh round of soul searching has broken out among Republicans about how to repair the GOP’s problems, a new Washington Post poll finds that public support for gay marriage — a key area where GOP Congressional officials continue to refuse to evolve — is at an all time high.

More tellingly, support for gay marriage is even higher among key voter groups identified in today’s big Republican National Committee report as constituencies the GOP needs to do a better job winning over — higher than among the public overall.

The Post poll’s toplines show that Americans think gay marriage should be legal by a 58-36 margin. Independents believe this by 62-33; moderates by 71-24. But Republicans oppose gay marriage by 59-34, and conservatives by 60-33. The Post polling team sends over these numbers among other core constituencies:

* Voters aged 18-29 support legalizing gay marriage by 81-15.

* Nonwhite voters support legalizing gay marriage by 61-32.

* College educated whites support legalizing gay marriage by 65-29.

This last constituency — socially liberal college educated whites, along with young and minority voters — is increasingly making up a key pillar of the Democratic coalition, as Dems grow less reliant on non-college whites. Democrats are increasingly less inclined to shape their agenda around the views and priorities of culturally conservative downscale white voters — even as Republicans are growing overly reliant on them.

Tellingly, non-college whites show significantly less support for gay marriage (53-42) than college educated whites do, and less support than the public overall. (A recent Quinnipiac poll also offered similar findings, showing that Latinos, young voters and college educated whites were significantly more supportive of gay marriage than overall Americans, while non-college whites oppose it.)

The Post poll’s findings among young and nonwhite voters are also significant. Today’s big RNC report on the health of the party doesn’t directly engage on the issue of gay marriage, but it does note that for “many younger voters…issues involving the treatment and the rights of gays” are a “gateway into whether the party is a place they want to be.” Meanwhile, the report also notes that if the party wants to appeal to minority voters, the party “must in fact and deed be inclusive and welcoming.”

The above numbers suggest that gay marriage is a good place to start. Marriage equality is an area in which we’re seeing an astonishingly rapid evolution in public attitudes. As such, it’s a good test of whether the GOP will be able to shed its image as hidebound and intolerant, to stop alienating key groups it cannot afford to alienate any further, and to evolve along with the changing culture and demographics of America.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.