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An improved sense that he understands voters' problems boosted Mitt Romney to victory in the Illinois primary, as did a less religiously focused, less strongly conservative electorate than he's faced in other contests, especially to the south. But a shortfall among less well-heeled Republicans marks his continued challenges.

Indeed exit poll results indicated that Romney owed his victory in Illinois to two groups: voters with more than $100,000 in household incomes and those with college degrees. Among those less educated, or less well-off, he only split the vote with Rick Santorum.

Other factors helped Romney. Six in 10 Illinois voters said he has the best chance of beating Barack Obama, better than his average in exit polls this year. And Romney narrowly led Santorum as the candidate who "best understands the problems of average Americans." It was only the second state, of seven where the question's been asked, in which Romney's prevailed on empathy. The other was Florida.

Among other advantages for Romney, the Illinois primary was characterized by far fewer evangelicals than most of the Southern contests and fewer voters seeking a candidate who shares their religious beliefs, two groups in which he's struggled. Forty-three percent were evangelicals, about half their share in Alabama and Mississippi last week. Nearly half in those states were highly focused on shared religious beliefs; it was just a quarter in Illinois, fewer even than in Ohio early this month.

Fifty-four percent of voters said they cared most about supporting either the candidate who's best able to defeat Obama or the one with the best experience, two strong groups for Romney. But, marking one of Romney's ongoing shortfalls, more than four in 10 cared most about the candidate with "strong moral character" or the "true conservative" - and in those groups Santorum easily won.

A class distinction underscored another continuing obstacle for Romney, one of the wealthiest people ever to seek the presidency. The exit poll, analyzed for ABC by Langer Research Associates, found that he beat Santorum by 18 percentage points among college-educated voters and by 25 points among those with incomes of $100,000 or more. Among the half who lack a college degree, Romney had 38 percent, Santorum 39. Among the more than six in 10 with incomes below $100,000, Romney had 38 percent, Santorum 40.

It helped Romney, then, that 37 percent of voters in the Illinois primary had $100,000-plus incomes, making this among the better-off electorates to date, particularly in comparison to some of the Southern states. But, should he capture the nomination, his comparative lack of appeal to less well-off voters could trouble him in a general election campaign.

Other results show some of the sharp differences between Republican and Democratic voters that may play out in November. In the Illinois Democratic primary in 2008, 57 percent of voters were white, and 32 percent were from Chicago, compared with 42 percent from the suburbs. In the Illinois Republican primary Tuesday, 98 percent were white, 69 percent were suburban and just 4 percent were Chicagoans.

Across all contests to date for which exit poll data are available, 93 percent of Republican primary voters have been white (it was 89 percent in 2008). Remove Florida and Arizona, and it's 96 percent. Whites, by contrast, accounted for 74 percent of voters in the 2008 general election.

If those are future concerns for the eventual GOP nominee, the party itself still is riven. Romney won just 35 percent of evangelicals, compared with 51 percent of non-evangelicals. Santorum again won "very" conservative voters, three in 10 of the total, here by a 15-point margin. Fifty-seven percent of voters were looking for someone who shares their religious beliefs - fewer than in the South, but a lot. And Santorum won them, on the strength of those who care about it "a great deal."

On Romney's part, what worked best was electability: Again more voters picked that as the single most important attribute in a candidate, and among those who selected it, a vast 71 percent supported Romney. That was his biggest share of "beat Obama" to date, save two states - Virginia (where he ran only against Ron Paul) and Massachusetts (where he used to work).

Among other exit poll results:

IDEOLOGY - A bit more than four in 10 Illinois voters said Romney is "not conservative enough," a criticism that's dogged him in the Republican electorate all year. But in this state as many said he's "about right" on the issues. This result looks much more like it did in Ohio, where Romney narrowly won the primary, and much less like it did in Alabama, Mississippi, Georgia and Tennessee, where far larger numbers branded him as insufficiently conservative.

Notable, too, is that among those in Illinois who called Romney "not conservative enough," 22 percent voted for him anyway - considerably better than his showing in this group in the six states where the question previously has been asked.

About as many said Santorum is "about right" ideologically as said that of Romney. But if Romney has challenges with very conservative voters, so has Santorum among moderates and liberals; a third in Illinois called Santorum "too conservative," and of those who said so, seven in 10 backed Romney. Among moderate and liberal voters combined - just more than a third of the electorate - Romney beat Santorum by 19 points.

That said, conservatives are ascendant in the state party: The number of Illinois Republican primary voters who identify themselves as conservative has grown steadily from 43 percent in 1992 to 50 percent in 1996, 61 percent in 2008 and 65 percent this year. "Very" conservatives, 15 percent in 1996, were twice that numerous today.

ENTHUSIASM and the LONG ROAD - The GOP electorate remains relatively unenthusiastic: Just fewer than half of Illinois voters said they strongly favored their candidate, roughly the average this year but below its highs in Alabama, Oklahoma and Iowa. Most of the rest liked their guy, but with reservations.

That said - love him, or just like him - two-thirds of Illinois GOP voters also said they were committed to their candidate winning, even if that means a long, drawn-out contest. Three in 10 said they'd like to see the race end as soon as possible, even if it means their candidate might not prevail.

By Gary Langer, with Gregory Holyk and Damla Ergun.