Rick Santorum and Mitt Romney have battled essentially to parity in basic popularity, far outpacing Newt Gingrich, who's faded to a new low in the 2012 election cycle. But Santorum's arc flattened this week, underscoring a potentially difficult battle ahead.

Within their party, 65 percent of Republicans express a favorable opinion of Romney overall, and about as many, 61 percent, hold a positive view of Santorum, levels that suggest many party regulars ultimately would accept either candidate as their nominee.

Gingrich, maybe not: He's dropped to 45 percent favorability within the GOP in this ABC News/Washington Post poll, produced for ABC by Langer Research Associates. That's down by 10 points in the past month and by 15 points from Gingrich's best, in November and December.

Still Santorum, after advancing earlier this month, saw his ratings flatten overall this week, with a 10-point increase in unfavorable views among Americans who describe themselves as "very" conservative. It's still among his best groups, though, and one in which he continues to lead Romney.

Santorum and Romney alike, moreover, remain well below the levels of popularity - both within their own party and among all Americans, including independents - that had been attained by this point in the 2008 campaign by eventual nominees John McCain and Barack Obama.

McCain and Obama both had broken through the 70-point favorability barrier in their own parties by January 2008, and were at or over 60 percent among political independents. Santorum, and Romney, by contrast, have just 33 percent favorable ratings from independents now.

Among all adults, while they're essentially tied in overall favorability, at 37 and 36 percent, respectively, Romney's unfavorable ratings exceed Santorum's by 10 points, 44 percent vs. 34 percent - meaning that, unlike McCain and Obama last time, neither is clearly above water. (More people have yet to form an opinion of Santorum - 30 percent, vs. 19 percent for Romney.)

The Republican contenders meet tonight for their 20 th debate, their first since Jan. 26, a face-off that preceded Santorum's surge and Gingrich's fade. It's their last debate before the Arizona and Michigan primaries next Tuesday.

TREND - The most striking recent trend, reported last week, was Santorum's rise in favorability among Republicans, from 48 percent in early January to 61 percent, marking his emergence as the latest non-Romney alternative after winning the Colorado and Minnesota GOP caucuses and the Missouri beauty contest primary.

This week, as noted, that surge stalled. Among very conservatives, Santorum had a 67-13 percent favorable-unfavorable rating last week; it's 61-23 percent now. Among conservative Republicans, last week Santorum had a 69-12 percent score; this week, 66-19 percent.

Santorum still does better than Romney's 52-38 percent among very conservatives. Among conservative Republicans, though, they're very similarly rated, with Romney at 69-23 percent.

A concern for Romney is his erratic trend line among independents. After essentially breaking even in popularity with these customary swing voters last week (37-40 percent), he's at a 14-point disadvantage this week (33-47 percent), halfway back to his January level. Santorum's got challenges in this key group too, with a 33-38 percent favorability rating.

Whatever their difficulties, both Santorum and Romney are in enviable positions compared with Gingrich. More than twice as many Americans now see him unfavorably as favorably, and a third have a "strongly" negative view of him, about four times his strongly positive score.

Gingrich only manages parity among Republicans (45-42 percent) and among very conservatives (43-48 percent), with a 15-point increase in unfavorable ratings in that group since late January. Among conservative Republicans he's got a 50-37 percent differential, but that's vastly lower than his 64-27 percent score in January, as well as far behind Santorum and Romney alike.

METHODOLOGY - This ABC News/Washington Post poll was conducted by landline and cell phone Feb. 15-19, 2012, among a random national sample of 1,012 adults. Results have a margin of sampling error of 3.5 points. The survey was produced for ABC News by Langer Research Associates of New York, N.Y., with sampling, data collection and tabulation by SSRS/Social Science Research Solutions of Media, Pa.