Mitt Romney is comfortably poised to win the lion’s share of Illinois’ 54 delegates this evening. Based on his substantial polling lead, the preferences of urban and suburban Republicans, and outspending Santorum 21-1 in the upstate media market, it’s likely that the frontrunner will win the 12 congressional districts of Chicagoland, leaving Santorum fighting for the other five.

Nate Silver recaps:

A week ago, Mr. Santorum seemed to have a decent shot in Illinois. He was down by just four points in The Chicago Tribune’s poll, which has had a strong track record — and that was before his wins in Alabama and Mississippi, which got him some favorable news coverage. The demographics of Illinois aren’t terrific for Mr. Santorum, but almost half of the Republican vote there is outside the Chicago metropolitan area, and downstate Illinois should be friendly terrain for him.

The polls have broken sharply against Mr. Santorum in Illinois, however. He trailed Mr. Romney by 14 and 15 points in two polls conducted over the weekend there and is behind by a similar margin in our forecast average, which gives him a 3 percent chance of winning the state.

As for the other candidates, Newt Gingrich appears to be broke and on vacation, and Ron Paul is nowhere to be seen, though his supporters are making quite a scene in Washington and Missouri, both caucus states with non-binding primaries.

The real action in Illinois today is an unusual primary battle in the northern 16th Congressional District, where the Tea Party-backed ten-term incumbent Don Manzullo (R-Ill) is running against freshman Adam Kinzinger (R-Ill), who has been backed by Majority Leader Eric Cantor’s (R-Va) Young Guns PAC.

Redistricting meant Kinzinger faced the Morton’s fork of challenging either Jesse Jackson, Jr. (D-Ill) in a liberal district or taking on Manzullo, a veteran conservative. Riding a wave of Tea Party support into office in 2010, the young Kinzinger failed to join the Tea Party Caucus once he got there, and his critics say he’s gone soft on cutting spending. His apostasies have been a boon to Manzullo, who has been endorsed by several notable conservative organizations including Freedom Works, the ACU and RedState.

David Catanese reports:

These conservative groups are attempting to frame the first GOP member-on-member battle of the 2012 cycle as a test of ideological purity, arguing that in 14 short months, Kinzinger has lost his way. They cite his timidity on votes to cut discretionary spending and his abandonment of the conservative Republican Study Committee in favor of the more moderate Tuesday Group. A Sarah Palin endorsee in 2010, Kinzinger declined to join the Tea Party Caucus once he got to Washington.

“We backed some guys last time that are a little disappointing, who took votes we don’t support. This is an example of a guy who definitely strayed,” said Brendan Steinhauser, FreedomWorks’s campaign director. “It’s a continuing struggle between the tea party movement and the establishment.”

Among some members of the Illinois delegation, there’s a sense that Manzullo could’ve avoided this bloody intraparty fight by retiring. “Here’s a guy with a bright future, clearly people see a lot in him. It’s like, you’ve been in 20 years, let the young guy go have his shot,” said an Illinois operative unaligned in the race.

Cantor has put a significant amount of effort into supporting young Republican House members and they comprise a large chunk of his support. Backing Kinzinger is a way to demonstrate that he’s true to his word (he’s already framing it that way) and if Kinzinger wins, that he wasn’t afraid to take sides in tough primaries.  If Manzullo wins, however, there will be bridges to mend. The Illinois congressman told The Hill on Monday that Cantor “needs to step down as majority leader” for his endorsement.