This is the kind of poll that should be pretty discouraging to Jon Huntsman, Newt Gingrich, and Rudy Giuliani.

Mitt Romney’s religion is far less of a concern to potential Republican voters than it once was, and a candidate’s race or sex are non-issues for vast majorities. But if a GOP contender supports gay civil unions — or has committed marital infidelity — all bets are off.

Twenty percent of Republicans and Republican leaning-independents in the latest ABC News/Washington Post poll say they’d be less likely to support a candidate who’s a Mormon; that compares to 36 percent when Romney entered the national political stage in December 2006.

Far more, 54 percent, say they’d be less apt to support a candidate for president who’s been unfaithful to his or her spouse. And 50 percent would be less apt to support a candidate who favors civil unions for gay couples.

This part of the poll was less about individual candidates, and more about candidate attributes. Respondents were told, “I’m going to read a few attributes that might be found in a candidate for president. Please tell me if each would make you (more likely) to vote for that candidate for president, or (less likely) to vote for that candidate, or if it wouldn’t matter.”

The one thing GOP voters said they’re most troubled by adultery — if a candidate “had an extramarital affair,” 54% of Republicans are less likely to support that candidate. This would seemingly be a deal-breaker for Gingrich and Giuliani, except (a) these two are doomed anyway; and (b) primary/caucus voters would have to hear about this part of their background for it to matter. (John McCain was the first admitted adulterer to ever win a major party presidential nomination, but the issue didn’t affect him because neither the media nor the other candidates ever alerted the public.)

The next major concern is a candidate who supports civil unions for same-sex couples — 50% of Republican voters said they’re less likely to back a candidate with the “wrong” position on this.

After that, there’s a significant drop-off, with 20% balking at a Mormon candidate and 23% balking at former Obama administration officials.

Looking at the specific candidates, this should be a fairly significant wake-up call for Huntsman — of the attributes GOP voters don’t like most, he gets a check mark next to three of the four. He supports civil unions; he’s a Mormon; and he worked for Obama. It would appear, then, that his candidacy is over before it starts.

As for Romney, it’s probably reassuring that anti-Mormon animus has dropped in recent years, but the poll isn’t exactly heartening for him, either. One in five Republicans is “less likely” to support a Mormon candidate, all other things equal. In a close contest, if the nomination comes down to Romney, Pawlenty, and perhaps Rick Perry, if a fifth of the party has a problem with Romney’s religion, it could mean the difference between success and failure.

What’s more, the actual number may be even higher. One in five were willing to admit this anti-Mormon attitude to the pollster who called their home, but what about those Republicans who have a problem with this minority faith, but felt embarrassed to admit it out loud on the phone?

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Steve Benen

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.