On Saturday, a lot of people were wondering how much longer Rick Santorum would stay in the Republican presidential race. Tonight, he has won Missouri and likely Minnesota and he's reemerged as the leading conservative alternative to Mitt Romney. Momentum, conventional wisdom holds, is now on his side. But if there's one thing we've learned so far in this volatile 2012 race, it's that momentum isn't as powerful a force as it used to be.

Typically, political analysts view primaries as sequential contests in which a candidate's performance in one dictates how he or she will do in another. Once one candidate builds up a head of steam, he or she is in a strong position in the next contests. Conversely, a weak showing is supposed to spell doom for a candidate as the primaries go on. However, it hasn't exactly worked out that way this time.

After Santorum won Iowa, he had underwelming performances in successive contests in New Hampshire, South Carolina, Florida and Nevada. By contrast, Gingrich finished fourth in both Iowa and New Hamphsire and was dead in the water heading into South Carolina. But he came back to trounce Romney there, who had been coming off his huge win in New Hampshire. Despite being blown out in South Carolina, Romney bounced back to run away with the race in Florida and Nevada, and some people expected him to run the table in February. Yet here comes Santorum again.

So, it's perfectly possible that Santorum has now supplanted Gingrich as the anti-Romney given the momentum coming out of tonight. But if we take any lessons from the race so far, we probably should be cautious about placing too much importance on momentum.