The debate that's raged inside the Republican Party in the Obama era gets its biggest test tonight with the Iowa caucuses, with voting that will provide a key indicator of rank-and-file sentiments in a year of big opportunity for the GOP.

The race boils down to pragmatism vs. purity. It will play out on ballots at precinct caucus sites in Iowa as Mitt Romney vs. everyone else.

Romney is expected to fare well in the caucuses, so well that he could very well be a winner even if he finishes second or third tonight. That's the cushion he's built himself by managing expectations expertly , and building up a huge lead in next-up New Hampshire.

But even under the best of circumstances for Romney, some two-thirds of Republicans are likely to cast ballots for someone other than the frontrunner.

That could mean a victorious Rick Santorum, consolidating the evangelical vote. It could mean Ron Paul winning on the strength of his insurgent brand of libertarianism, or even a surprise showing from the up-again, down-again, just-maybe-up-again Newt Gingrich.

What it means in any of those events is a challenger for Romney. The former Massachusetts governor has come to represent the establishment of the Republican Party, with inconvenient policy shifts to add to a resume that's troubling to many rank-and-file conservatives.

Romney is the perfect candidate on paper - tested by a previous campaign, looking and sounding the part, almost certainly the strongest match-up against President Obama in an election that's likely to be dominated by jobs and economic issues.

But those slips of paper in caucus sites throughout Iowa have a funny way of making that other paper look worthless in a hurry.

The struggle that's defined the GOP in the ascendance of the tea party movement, all through its flirtations with a series of reluctant presidential candidates, will continue to define a race that won't be over any time soon.