Mitt Romney's rebounded among strong conservatives since his wins in the Michigan and Arizona primaries, potentially bolstering his prospects in the race for the Republican nomination. But he and his GOP opponents face continued challenges in popularity more broadly.

All four Republican contenders remain underwater in overall favorability in the latest ABC News/Washington Post poll, marking the difficulties the survivor may face against Barack Obama. More Americans hold negative than positive views of Romney by a 10-point margin, Rick Santorum by 8 points, Ron Paul by 9 points and Newt Gingrich by a whopping 33 points.

Among customarily swing-voting independents, moreover, all but Paul is seen more unfavorably than favorably in this poll, produced for ABC by Langer Research Associates. And Paul's got trouble, as does Gingrich, within the GOP itself.

Overall ratings are little changed from previous measurements. Notably, though, Romney has improved among both "very" and "somewhat" conservative Americans, as well as among conservative Republicans in particular - up by 12, 10 and 11 points in these groups, respectively, versus a week ago. His gain among very conservatives comes from the ranks of the undecided in this group; among somewhat conservatives, by contrast, negative views moved to positive ones.

Indeed Romney, after falling to new lows among conservatives before the Michigan and Arizona contests, has moved back to parity with Santorum among some conservative groups, notably conservative Republicans. In this group, 67 percent now see Romney favorably; Santorum, 68 percent.

Slight shifts in group sizes largely wash out Romney's changes overall. Among all adults, 35 percent see Romney favorably, 45 percent unfavorably, essentially unchanged from his 33-46 percent score last week. Among groups, Romney's popularity is 11 points higher among seniors than among adults under 30, and also 11 points higher among people with mid-level or higher incomes than among those earning less than $50,000.

Santorum shows little change both overall and within groups, with a 32-40 percent favorable-unfavorable rating now compared with 34-37 percent last week. Santorum, who's made some controversial comments about the role of women in society, has a gender gap - just 28 percent of women see him favorably, versus an also-weak 36 percent of men.

Paul's ratings, 32-41 percent favorable-unfavorable, are essentially the same as when last measured in an ABC/Post poll in early January. He's at 38-35 percent among independents, better than his rivals; but, at 38-44 percent among Republicans, numerically the worst of the lot within the GOP. Among other groups, Paul does markedly better with college-educated adults (41 percent see him favorably) than among those who have not been graduated (28 percent see him positively). And he has a larger gender gap than Santorum - 39 percent favorable among men, 25 percent among women.

Deepest into the sea is Gingrich - 23 percent of Americans see him favorably, a new low this election cycle; 56 percent unfavorably, a new high. Republicans essentially divide on Gingrich, but independents see him negatively by a vast 58-21 percent.

Part of the Republican candidates' problems might reflect the fact that they're in the midst of a hard-fought campaign, which is not always a pretty sight. At the same time, as noted in previous reports, all are well below the levels of popularity attained by previous candidates - John McCain and Obama among them - at about this stage of their own nomination battles.

METHODOLOGY - This ABC News/Washington Post poll was conducted by landline and cell phone Feb. 29-March 4, 2012, among a random national sample of 1,014 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.