Early exit polls show Mitt Romney's win in the Illinois Republican primary rests on broad leads among voters with higher incomes and more formal education, with a boost from those focused on defeating President Barack Obama in November.

KEYS TO THE MIDWEST: As in Ohio and Michigan, where Romney eked out narrow victories over Rick Santorum, the two candidates ran about evenly among those with lower incomes and less formal education. But Romney outperformed his showing in the other two Midwestern states among higher income, higher education voters. Romney more than doubled his margin over Santorum among college graduates, and nearly did so among those with incomes above $100,000.

ELECTABILITY PROPELS ROMNEY: Romney carried more than 70 percent of the vote among those who said it was most important to choose a candidate who could win in November. Aside from Massachusetts and Virginia (where only Romney and Ron Paul were on the ballot) that is Romney's best performance among this group.

TEA PARTY BACKERS: Romney holds a slim advantage over Santorum among tea party backers. The two split the group about evenly in Ohio and Michigan, and Romney has only won them by a significant amount in five states where exit or entrance polls have been conducted.

A DIFFERENT RELIGIOUS COMPOSITION: About 4 in 10 voters were white born-again Christians, well below the levels seen in last week's primaries in Alabama and Mississippi, but above the 33 percent of Illinois GOP primary voters who were white evangelicals in the 2008 exit poll. These voters broke for Santorum, as did those who said it was deeply important that a candidate shares their religious views.

But Santorum again did not appeal as strongly to his fellow Catholics as he did to evangelicals. Overall, more than a third of Illinois GOP voters were Catholic, among the highest levels seen across 11 states where GOP voters were asked their religion. Romney won about half the vote among Catholics compared with about a third for Santorum. And while Santorum held a wide lead among evangelical Christians who attend services weekly, among Catholics who do the same, the vote was about evenly split between Santorum and Romney.

SANTORUM STRONG AMONG CONSERVATIVES: Among the 3 in 10 voters who called themselves "very conservative," Santorum held a double-digit lead. However Romney won about half of the vote among all others, about 20 points ahead of Santorum.

Among those voters who said it was most important to choose a true conservative, Santorum held a 7-to-1 margin over Romney. About 4 in 10 voters overall in Illinois said Romney's positions on the issues were not conservative enough, but the former Massachusetts governor still carried about one-quarter of the vote among this group. Santorum, by contrast, won just 6 percent of the vote among those who considered his positions too conservative.

ECONOMIC VIEWS: Most Illinois voters called the economy their top issue in choosing a candidate, while a quarter called the deficit tops. About half of voters in both groups said they supported Romney. Nearly half of voters said the nation's economic conditions were getting worse and another third said they were staying about the same. Just 1 in 5 saw the economy as improving.

Economy voters were a bit more likely to say they were seeking a candidate with the right experience, and that was another group Romney dominated. Among those who said a candidate's experience was the most important factor, Romney held a nearly 50-point lead.

NOT SEEKING A QUICK END: About a third of those who voted Tuesday said they wanted to see the nomination campaign end quickly. Instead, most favored a long contest with their favored candidate prevailing in the end. About a third of Romney backers wanted things to end quickly, compared with one-quarter of Santorum backers.

Preliminary results from the survey of 1,621 Illinois Republican voters have a margin of sampling error of plus or minus 4 percentage points. The exit poll was conducted for The Associated Press and the television networks by Edison Research at 35 randomly selected polling places around the state.