(Update Jan. 4, 2011: Romney wins Iowa by a nose! Watch the race to see the photo finish.)

With the Iowa caucus on Tuesday, we’ve reached that time in the campaign cycle when it dawns on the media that the horse race they have been covering all along—candidates’ standings in national polls—is not the one that counts. A national lead is nice, but it doesn’t mean much unless a candidate can win delegates state by state (just ask Rudy Giuliani). And while the nomination is rarely won in Iowa, it can be lost there (as Howard Dean could tell you). So who will win the caucus? Romney, running slow and steady along the rail? Paul, peaking at the right time? Santorum in a sudden surge? No one knows—especially since statewide polls are notoriously fickle—but that won’t stop us from enjoying the race. (For Slate’s animation of the national GOP horse race, click here.)

In the above animation, each horse is a candidate, with its horizontal position representing that candidate’s standing in the Iowa polls. (Mouse over any horse for a picture of the corresponding candidate.) Our starting line is January 2011, when the first major polls of Iowa voters began. The horse’s position is the determined by the average of the last three polls, to smooth out the margins of error. The data is adapted from Real Clear Politics’ aggregation of Iowa polls.