Charlie Riedel/AP Photo

Analysis

After a string of Tuesdays where Mitt Romney had something to prove, it's time for Rick Santorum to deliver in the Land of Lincoln.

Yes, a Romney loss in Illinois tomorrow would be an embarrassment of epic proportions. Yes, Romney simply must win the state to keep intact his argument that he's a shoo-in for the nomination.

But that's the point. Santorum is down too far in the delegate count to win only the states he's expected to win.

See the Primary Scorecard Here .

Santorum needs an upset. He needs it to come in a big state, and fast, for him to change the dynamics of the race sufficiently for him to actually win.

Why Illinois? For starters, it's next on a calendar that isn't particularly friendly to Santorum these next few weeks. After Louisiana votes this weekend, Maryland, Wisconsin, and the District of Columbia (where Santorum isn't even on the ballot) are up next, and will be tough for Santorum to break through in.

Illinois also happens to be a large state, awarding 69 convention delegates, in a region Santorum is claiming as his own. He said over the weekend that he's "won every state in the Midwest" - not even close to technically true, since he lost Ohio and Michigan - but he has fared well in Iowa and in Missouri's non-binding primary.

But it's mostly what Illinois is not that makes it a potential breakthrough moment for Santorum. It's not a Southern state with a large share of evangelicals voting in the GOP primary. It's also not a caucus state, the kind of lower-turnout affair that has fueled Santorum's momentum in previous contests.

Plus, voting in Illinois takes place as Santorum has something to prove. His decision to burn two gaffe-filled campaign days in Puerto Rico, where he secured just 8 percent of the vote and zero delegates yesterday, may stand as one of the biggest miscalculations of the election.

Meanwhile, Romney has been able to pad his delegate lead, powered by wins in low-turnout US territories, despite higher-profile wins by Santorum in Alabama, Mississippi, and Kansas.

A win in Illinois would be a different story. It may be Santorum's last best shot to prove that he's a contender, not merely a potential spoiler.