The polls going into today show an effective three-way split among Mitt Romney, Newt Gingrich and Rick Santorum in both states. What’s already clear is that Romney has trouble getting even a third of the vote in Southern primaries. He’s been no higher than 28 percent in Southern states up to now. (I don’t count Florida or Virginia, whose politics are increasingly influenced by Northern migrants. In Florida, for example, Romney lost in the Panhandle, the most culturally “Southern” part of the state. And Ron Paul was his only opponent in Virginia.) Romney would count it as a victory to break through to a third of the vote in either of these states.

Romney could thus have a great night if Gingrich and Santorum closely split the remaining, more conservative vote. Or Romney could lose twice if one of his two rivals could consolidate just enough of the conservative vote to win. And the difference between these two outcomes could be very small — a couple of percentage points.  (That’s why I draw on the title of one of Matt Miller’s excellent books in my headline — the book has nothing to do with primaries.)

Clearly Santorum needs to get Gingrich out of this race. Thus, the most important question tonight may not be how Romney does, but whether Gingrich manages at least one victory. If Gingrich loses both states, he has indicated he will stay in the race anyway. But his rationale for staying in will be greatly weakened and conservative activists will increase the pressure on him to leave. Santorum really needs a one-on-one against Romney in the Illinois primary this month. If Gingrich wins just one primary, he can hang on and continue to make life more difficult for Santorum.

Santorum needs at least one victory tonight. Two losses for him would severely damage his claim to being the true conservative alternative to Romney.

Romney has the least to lose and the most to win. If he wins even one of these states narrowly, he will get lots of good press for “breaking through” in the South. If he loses both, he can go on just fine — especially if he loses one state to Gingrich and the other to Santorum. A double Santorum win would be the biggest problem for Romney.

It could all hang on just a couple of percentage points of the vote, and make no mistake: Every analyst, including this one, will make a big deal over those few percentage points. One last thing: Don’t rule out someone breaking away in one or both of these primaries. Close polls on election eve don’t always translate into a close result on voting day. If you look at past performance, the candidate with the best chance of running ahead of his poll numbers is Santorum.