A strongly conservative, religiously inspired Republican electorate turned the tables on Mitt Romney in Louisiana, supplying Rick Santorum with both an easy win and a sharp riposte to Romney's victory in Illinois four days earlier.

Santorum was boosted by one of the highest concentrations of very conservative voters in any primary this year. Evangelicals, while less dominant than in most other Southern states, backed him by a record margin over Romney. And Santorum's tally was his best to date among voters focused on supporting a candidate who shares their religious beliefs.

Even in this inhospitable environment, Romney again prevailed as most likely to defeat Barack Obama in November, by 43 to 33 percent vs. Santorum. But that was Santorum's best score on this question in any of the nine contests to date in which it's been asked. In previous races, voters picked Romney as more electable by a 32-point margin; in Illinois, by 38. In Louisiana, Santorum shaved that to 10.

The exit poll, analyzed for ABC by Langer Research Associates, found that defeating Obama was the most important attribute to a plurality of voters, 37 percent - almost as many as in previous GOP contests combined - and those voters favored Romney by 20 points, 49-29 percent. But that was half Romney's usual margin over Santorum among beat-Obama voters; in Illinois, an especially sharp contrast to Louisiana, Romney won them by a blowout 57 points, 74-17 percent.

As well as improving among voters who were chiefly focused on beating Obama, Santorum powered ahead in Louisiana with record support levels among those who cared most about moral character or the "true conservative" in the race, beating Romney by a remarkable 64 points in these two groups.

In another change from Illinois, Louisiana voters picked Santorum and even Newt Gingrich over Romney, by 41-25-21 percent, as the candidate they think best understands the problems of average Americans; Santorum had won that comparison in several previous contests (less widely than in Louisiana) before seeing Romney wrest it away last Tuesday.

In addition to his clear resonance with Louisiana voters, Santorum may have benefited from another dynamic, the fading candidacy of Gingrich even in the South; his vote share Saturday was roughly half its level in Alabama and Mississippi. Had Gingrich and Ron Paul not been in the race at all, the exit poll indicated that Santorum would have gained 12 points, Romney 8.

Nonetheless, the results also underscore vulnerabilities for Santorum. The concentration of his support among especially conservative, religiously focused voters works only in states in which they predominate. And Santorum continues to lag on experience: While just 14 percent in Louisiana prioritized this attribute, he finished numerically third among them, behind Romney and Gingrich alike.

Santorum did prove in Louisiana that he can win without an overwhelming number of evangelicals; they accounted for 59 percent of voters on Saturday, compared with levels in the 70s and 80s in other states he's won, apart from the Iowa caucuses.

One reason, as noted, is that while evangelicals were fewer in number, he won them by an outsized margin, 54-21 percent, his best to date in this group. Another is that Catholics accounted for a larger-than-usual share of the electorate in Louisiana, and Santorum crushed Romney in particular among highly religious Catholics.

Religiously inspired voting more generally ran high in the state. Sixty-four percent of primary voters said they go to church at least weekly; they backed Santorum by 56-24 percent, while less-frequent (or non-) churchgoers much more narrowly favored Romney, 39-28 percent. More, 73 percent, said it mattered to them to support a candidate who shares their religious beliefs, and 41 percent said it mattered a great deal - more than said so in Illinois by 17 and 18 points, respectively.

Santorum won all voters focused on shared religious beliefs by 55-25 percent and those focused on this a "great deal" by 66-16 percent, both record margins in exit polls to date.

Emphasizing Santorum's dominance in Louisiana, Romney won just one group within income and education categories: voters earning more than $200,000 a year. He even lost seniors, usually a Romney stronghold.

Another result shows the difficulty Romney faces in winning over elements of Santorum's support base. While 54 percent of Romney's voters said they'd be satisfied with Santorum as the nominee, fewer Santorum voters, 42 percent, said they'd be satisfied with Romney.

Finally, there's the Etch A Sketch comment, in which Romney's communications chief this week likened the November campaign to the children's toy: "You can kind of shake it up and we start all over again." Just one in five Louisiana voters called the controversy over the remark important to their vote. But those who did voted for Santorum over Romney by a 36-point margin, 55-19 percent.

By Gary Langer, with Gregory Holyk and Damla Ergun.