Illinois provides the latest showdown in the Republican presidential nomination race, and a second poll puts Mitt Romney up by double digits over his main competitor Rick Santorum.  Echoing a PPP poll taken at roughly the same time, the new ARG survey of 600 likely Illinois voters shows a 14-point lead for Romney, 44/30, with Newt Gingrich trailing badly at 13%.  But should both be taken with grains of salt?

Mitt Romney leads the Illinois Republican presidential primary with 44%. Romney is followed by Rick Santorum with 30%, Newt Gingrich with 13%, and Ron Paul with 8%.

Romney leads Santorum 45% to 35% among self-identified Republicans, followed by Gingrich with 12% and Paul with 4%. Among self-identified independents and Democrats, Romney leads with 42%, followed by Paul with 20%, Gingrich with 17%, and Santorum with 16%.

Romney leads Santorum 51% to 36% among likely Republican primary voters saying they have voted in early voting or by absentee ballot. Romney leads Santorum 43% to 30% among those saying they will vote on March 20, followed by Gingrich with 15% and Paul with 8%.

Let’s not forget that both PPP and ARG didn’t exactly cover themselves in glory when it came to polling Mississippi.  Four days before the primaries, ARG had Santorum 15 points off the lead, with Gingrich edging Romney 35/31 and Santorum at 20%.  The day before the primary, PPP had Santorum trailing by six in third place.  Santorum ended up winning Mississippi by two points and 4500 votes.  That counts as a pretty big swing and a miss, even in primary polling.

Illinois isn’t Mississippi, however.  Santorum won in the conservative southern state by winning the very conservative voter demo, which accounted for 42% of all primary voters.  That demo isn’t likely to be as strong in Illinois, which makes it more difficult for Santorum to outperform the polls.  ARG doesn’t split the vote by ideology, but it does look at Tea Party supporters as a separate demo.  Romney leads Santorum by six points among those voters, giving even less hope for another election-day surprise — again, though, assuming ARG has a representative sample in place, an assumption that didn’t turn out to well last week.

The unique nature of this primary might impact turnout models more than one would think, too.  As Eric Ostermeier points out, Illinois rarely has a meaningful presidential primary — or a competitive one, either:

Illinois has held its primary in March (1972 through 2004) or February (2008) in each of the last 10 presidential election cycles, but the state has done little to prolong the Republican Party’s nomination battles during this span.

A Smart Politics review of the 25 Illinois GOP presidential primary elections conducted since 1912 finds that only one contest has been decided by less than 10 points, with the winner enjoying an average victory margin of 60 points.

To say that there has been little drama in Illinois Republican primaries in recent decades is an understatement to be sure.

Overall, the winner of the state’s 25 primaries has averaged 77.0 percent of the vote with an average victory margin of 59.8 points.

Even after excluding the reelection campaigns of Ronald Reagan in 1984 and George W. Bush in 2004 – in which these incumbents did not face any primary challengers – the winner of the Illinois GOP primary has still averaged 75.0 points and a margin of victory of 56.3 points.

In comparison, a 14-point spread is nearly even money.  That kind of a close race, along with some downticket contests that might spark unusual turnout, makes previous Illinois primary models suspect.  However, bear in mind that Santorum won’t qualify for ten of the delegates tonight, and that even a narrow victory will mean that Romney will pad his delegate lead.