From Gregory Holyk and Gary Langer:

With the stage set for their latest debate, Herman Cain is challenging former Massachusetts’ Gov. Mitt Romney and Texas Gov. Rick Perry in overall popularity and leading both in strong sentiment among very conservative Americans – a key Republican constituency that’s backed away from Perry in the past month.

The question raised by the latest ABC News/Washington Post poll is whether Cain proves any better able than Perry to withstand the new level of scrutiny he faces, in particular among the sizable numbers of Americans who’ve yet to form an opinion of him either way.

Within their party, all three are popular: Romney is seen more favorably than unfavorably by 57-21 percent, Cain by 52-17 percent, Perry by 49-24 percent. Perry’s unfavorable rating has risen 13 points in his own party since last month. Views of Romney have held about steady among Republicans overall.

Especially notable is the intensity of sentiment among Republicans and political independents who describe themselves as “very” conservative. Thirty-three percent in this group see Cain “strongly” favorably, compared with 15 and 14 percent for Perry and Romney, respectively. It’s a key group, given that greater ideological commitment suggests higher turnout in GOP primaries.

Moreover, while Cain leads in strongly positive sentiment in this group, overall unfavorable ratings of Perry and Romney have advanced in the last month among very conservative Republicans and independents, by 16 and 12 points respectively – negative trends for both.

Specifically, 28 percent of very conservative Republicans and independents now see Perry unfavorably, up from 12 percent a month ago. That’s now about as many as have an unfavorable view of Romney, who’s been consistently weaker among very conservatives. Thirty-two percent in this group now see him unfavorably, up slightly from 20 percent.

Most of the Republican contenders meet in Las Vegas this evening for their fifth debate since Perry joined the race. It’s their first face-off since Cain vaulted to the upper echelon in GOP preferences, suggesting it may be his turn in the hot seat.

OUTSIDE – Outside their party, naturally, the GOP candidates are less popular – Democrats tend to see them negatively; independents roughly divide on Romney and Cain, while taking a dimmer view of Perry. Views among independents are especially critical in a general election campaign – they’re the quintessential swing voters.

Overall, given these divisions, the public roughly splits on Cain; 31 percent of Americans see him favorably, 27 percent unfavorably, with a plurality, 42 percent, unready to express an opinion. Romney’s in roughly the same place in terms of favorability (33 percent), but gets more negative reviews – 36 percent unfavorable, 9 points higher than Cain’s.

Perry, given his comparative weakness in the center, is in tougher shape; more Americans see him unfavorably than favorably by a 12-point margin, 38-26 percent, with unfavorable views 7 points higher than a month ago.

Favorability is the most basic measure of a public figure’s popularity. ABC and the Post will be tracking these ratings across a variety of individuals and entities throughout the next year, with ABC’s participation in the project directed by Langer Research Associates of New York.

Cain’s positioning is challenging for Romney on two fronts: As noted, Cain does better in intensity among very conservatives, who matter in the primaries; yet he also rivals Romney in more centrist constituencies that can be essential in a general election: In addition to independents, the two are rated similarly among moderates and “somewhat” conservatives alike.