Polling indicates that Mitt Romney appears to be charging toward a big victory in Tuesday’s Republican presidential primary in Illinois, a state he lost to John McCain by 18 percentage points four years ago. The state’s swing in Romney’s favor has less to do with changes among the state’s voters and more with his emergence as this year’s mainstream and moderate Republican choice.

You can catch exit poll analysis from the Illinois primary tonight by following the Post Politics live blog and @postpolls on twitter.

Illinois’s run-of-the-mill electorate stands to benefit Romney on both ideology and religiosity. Romney has lost every contest this year in which evangelicals made up a majority of voters, according to exit polls. But he’s won every contest where they account for less than half the electorate, as they did four years ago in Illinois. The story is similar for strongly conservative states: Romney lost Oklahoma, Mississippi and Tennessee, three states with among the highest percentage of “very conservative voters.”

Romney’s strength among ideologically moderate voters in the party contrasts with his appeal four years ago, when McCain was seen as the mainstream and most electable candidate. Indeed, very conservative voters were one of Romney’s best groups in Illinois. He won 37 percent of their votes, according to exit polling, similar to 36 percent for Mike Huckabee but about double the support of McCain. McCain won the contest by racking up large margins among “somewhat conservative,” moderate and liberal voters, who together made up over 70 percent of the electorate.

Other key factors to watch:

Finish up, please? Romney is in prime position to expand his delegate lead, making it harder for his competitors to argue they have a chance to forge a comeback. Watch exit poll results tonight for signs that Republicans are weary with the campaign slog.

Will Rick Santorum beat his polls again? An American Research Group poll and an automated survey by PPP (D) both show Romney with a double-digit lead in Illinois, but Santorum has consistently performed better than his poll numbers predicted. This phenomenon may be muted in Illinois for at least one reason. The Huffington Post’s Mark Blumenthal finds a correlation between Santorum’s outperformance and the percentage of evangelicals in the state. As noted earlier, Illinois was about average in the percentage of evangelicals in 2008, while Alabama and Mississippi set records for the most evangelical electorates this year.