The Republican presidential candidates spent the past week honing their Southern accents and talking awkwardly about their love of grits. With their Southern swing behind them, they will return to the industrial heartland for a potentially pivotal primary Tuesday in Illinois.

Illinois is the biggest of the three contests that will be held over the next week, after Saturday’s Missouri’s caucuses and Sunday’s Puerto Rico primary. Given Rick Santorum’s strong performance in the South, Illinois becomes the latest state where pressure will be on Mitt Romney to regain momentum. It also offers the former Massachusetts governor the opportunity to extend his winning streak in Midwestern primaries.

Romney struggled mightily to beat Santorum in primaries in the two other big Midwestern states, Michigan and Ohio. A Romney victory in Illinois would puncture Santorum’s hopes of winning and give the former governor a decisive hold on the GOP nomination.

Romney is only narrowly ahead in Illinois, according to the most recent public poll. But that puts him in better shape than he was at comparable times in Ohio and Michigan. In both of those states, he trailed Santorum a week out from the primary and used money and muscle to win.

The poll, by the Chicago Tribune and WGN-TV, showed Romney with 35 percent of the vote, followed by Santorum with 31 percent, former House speaker Newt Gingrich (Ga.) with 12 percent and Rep. Ron Paul (Tex.) with 7 percent. But that survey might understate Romney’s potential strength, according to a number of Republican strategists.

Romney’s edge

“We start out in Illinois ahead,” said a Romney supporter, who spoke on the condition of anonymity to talk candidly about proprietary data. “In both Ohio and Michigan, we started out behind and by double digits in both states. Romney had to come from way back to catch Santorum in both states. Here, he doesn’t have to do that.”

Santorum is at a disadvantage in the delegate fight, having failed to qualify a full slate of delegates on Illinois’s primary ballot. That hurt him in Ohio as well.

Economic issues are likely to dominate the primary campaign in Illinois. The state’s unemployment rate is above the national average and higher than in Michigan or Ohio. The state faces significant budgetary problems, despite a tax increase enacted a year ago, and its bond rating is one of the lowest in the country.

Despite the economy, Illinois is unlikely to be competitive in November given that President Obama’s home town is Chicago and that the state has supported Democratic nominees in the past five presidential elections.

But the primary provides another test for the Republican nominees to demonstrate their ability to motivate voters in the kind of big and diverse states that will determine the outcome in the fall — and to make the case about who is best equipped to defeat Obama nationally.

A state ‘in the mix’

Illinois Republicans find themselves in an unfamiliar position. The state hasn’t hosted a truly competitive Republican presidential primary in more than two decades. This year, their votes will probably help determine who runs against Obama in November.

“Illinois is surprised and delighted to be in the mix,” said Rep. Peter Roskam (R-Ill.). “It’s been a long time since that’s taken place.”

That doesn’t mean interest is running high — at least not yet. Former governor Jim Edgar said there has been more talk downstate over last week’s firing of University of Illinois basketball coach Bruce Weber than about the presidential race. “I can’t say there’s a lot of enthusiasm for anybody yet,” said Edgar, who has not endorsed a candidate.

Edgar said he was in South Carolina during its primary in January. “There’s no comparison,” he said. “That’s all you heard down there. It hasn’t gotten to anything like that.”

That could begin to change with Tuesday’s results from Mississippi and Alabama, and as the candidates begin campaigning in Illinois the rest of the week. Paul is scheduled for what his advisers predict will be a big rally on the University of Illinois campus Wednesday night. Gingrich has events scheduled Wednesday and Thursday, and Santorum will be in the state Friday.

Romney isn’t scheduled to be there this week as he campaigns elsewhere. But he and the super PAC supporting his campaign will have the heaviest presence on television through Tuesday. In Ohio and Michigan, he vastly outspent Santorum, only to win narrowly.

The turnout question

Just who shows up to vote Tuesday is one wild card. Because of redistricting, there are a series of competitive legislative and congressional primaries. That could draw a preponderance of party activists, many of them conservatives. Whether more moderate voters will show up is less clear.

The key for Romney will be to roll up a big vote in the collar counties around Chicago, as well as among Republicans in Cook County itself, where voters are more moderate. Concentrating on big cities and their suburbs was his winning formula in the other two Midwestern battlegrounds, particularly in Ohio, where his margins around Cincinnati and Cleveland overcame Santorum’s strength in small towns and rural areas.

More than half the Republican primary vote will come out of the Chicago area, and the Tribune-WGN poll indicated that Romney is doing best there. He was ahead by nine points in Cook County and 12 points in the Chicago suburbs. Santorum was running ahead, 35 percent to Romney’s 29 percent, in more conservative downstate Illinois, which refers to everything outside the Chicago area.

Having lost Michigan and Ohio, Santorum will need a victory in Illinois to prove his claim that he is the kind of Republican who can defeat Obama. But as the campaign begins in earnest, Romney remains the favorite to win Tuesday.