Mitt Romney's basic popularity looks more positive after his whisker-close win in the Iowa caucuses, while perceptions of Ron Paul are clearly negative for the first time in ABC News-Washington Post polling.

On the eve of the New Hampshire primary, 39 percent of Americans hold a favorable opinion of Romney, 34 percent unfavorable; numerically, his best net-positive score in ABC-Post polling since September. Paul, on the other hand, is seen favorably by 31 percent, but unfavorably by 38 percent, his most negative gap.

The other top scorer in Iowa, Rick Santorum, gets a more even split, but a numerically negative one: 27 percent favorable, 29 percent unfavorable. Many more adults, 44 percent, have yet to form an opinion of him one way or another.

The ratings matter because favorability is the most basic measure of a public figure's popularity. An overall positive rating is a key indicator of broad acceptability. This national survey, produced for ABC News by Langer Research Associates, tests the candidates beyond the confines of Iowa, where, out of about 122,000 caucus-goers, Romney and Santorum essentially tied (an eight-vote margin for Romney), with Paul third.

PARTY: Each of these candidates does far better in his own party than more broadly. Among Republicans, 61 percent see Romney favorably, 48 percent Santorum and 40 percent Paul. Their ratings are lower among independents, and far lower among Democrats.

Two results put Paul at a disadvantage: He's seen unfavorably by 39 percent of Republicans, essentially no fewer than see him favorably - in his own party. And Paul also lacks a positive score among independents, the key swing voters in national elections.

Santorum also is on the wrong side of favorability among independents, by a slight 6-point margin. Romney, on one hand, is rated more favorably than unfavorably by independents, by a 7-point margin. But his 61 percent favorable rating among Republicans is a tepid one, even if it beats Paul and Santorum's scores. President Obama, in an ABC-Post poll last month, had an 82 percent favorable rating among Democrats.

IDEOLOGY: While Romney has had trouble winning support from potential GOP voters who call themselves very conservative, he does have majority popularity - if not preference - in this group. His favorability rating is 57 percent among very conservative Republicans and independents, essentially the same as his 58 percent among those who are somewhat conservative.

Santorum matches Romney in popularity among very conservatives, but otherwise Romney is ahead of Santorum and Paul alike in these groups.

OTHERS: There are other differences among groups. Romney's popularity, for example, peaks among seniors, at 53 percent positive, compared with 29 percent among under-30s. Santorum also does much better with seniors - 45 percent favorable - than with younger adults.

Paul, by contrast, is most unpopular among adults age 50 and up; 47 percent see him unfavorably, compared with 25 percent of under-30s. Younger adults nationally don't see Paul more favorably, though; rather, more are undecided. (Young adults were his best age group by far in Iowa.) Paul also is 12 points less popular among women than men.

Romney does well, 47 percent favorable, among adults with household incomes more than $100,000 a year. And his favorability rating is more than 20 points lower among non-whites, 24 percent, than among whites, 45 percent. There are no significant income differences in favorability for Santorum and Paul, and the racial gaps, while present, are smaller.

Romney, finally, has a better favorability rating in the Northeast - 45 percent - than Santorum (34 percent) or Paul (32 percent). And a certain Northeastern state, of course, is where tonight's first-in-the-nation primary will be staged.

METHODOLOGY: This ABC News-Washington Post poll was conducted by landline and cell phone Jan. 4-8, 2012, among a random national sample of 1,013 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.