May, June, July, August, September.
Some very important developments took place at the very beginning of October. First, the field became completely settled as the remaining potential candidates announced they would not run: Chris Christie on October 4th and Sarah Palin on the 5th. Second, the Republican Party in Florida broke the national Republican Party's rules by pushing its primary up to January 31st. This caused all of the early primary states to push their dates forward as well. The result was that an entire month of the pre-Iowa primary season was eliminated. Instead of October opening with four months until Iowa, it was down to three.
The most important development that took place at the beginning of the month was that Christie officially declared he would not run. This shifted the entire balance of the race. Christie was the candidate most likely to win, since he had the unique ability to attract both establishment Republicans and Tea Partiers. Rather than entering the race and filling the vacuum, Christie's refusal to run made the void permanent: Republicans would never be satisfied with the field for 2012. Instead, they would search for the "least bad option."
But Christie was also the final obstacle in Mitt Romney's path to win the nomination. Mike Huckabee promised a populist, Southern, Evangelical alternative to Romney, but he did not run. Mitch Daniels and Tim Pawlenty both had the ability to serve as conservative "consensus candidates" who could satisfy (if not excite) both wings of the Republican Party. Daniels didn't run, and Pawlenty left the race prematurely. With Christie out, there were no longer any highly electable candidates in the race besides Romney (and Jon Huntsman, perpetually left in Romney's shadow).
With Romney a lock to win New Hampshire and the establishment wing of the Party, and with the rest of the field either unelectable or lacking conservative credentials of their own, Romney was set to win by default. On October 5th, Elephant Watcher was able to project that Mitt Romney would win the Republican 2012 presidential nomination.
The rest of October was marked by the continued slide of Rick Perry and Michele Bachmann, and the rise of Herman Cain. Perry, who had led the polls just a month earlier, sunk to fourth, fifth, and even sixth place in Iowa polls. Bachmann, who led during the summer, was essentially tied with Perry in single-digits in the state. But Cain rose to the top of both the Iowa polls and national primary polls.
Perry and Bachmann's crash also created an opening for Romney, who led or tied with Cain in later Iowa polls. Before October, Romney placed all of his resources in New Hampshire. Now, Romney's 2008 dream of a knocking out his competitors with a one-two punch in Iowa and New Hampshire looks like a realistic possibility--perhaps even a probability.
The two debates in October cemented Bachmann and Perry's status as second-tier candidates. Cain and Romney were resilient. But by the end of October, Cain was beginning to show signs of weakness. Unlike Romney and the previously-crashed candidates, Cain had never been challenged or vetted in any way. Under heavier scrutiny, Cain made a series of low-impact gaffes. His "9-9-9" tax plan became less popular the more people looked into it, prompting Cain to rethink the idea. And as October came to a close, a sexual harassment scandal from Cain's past came to light, with inconclusive results. Anti-Romney voters already cast about once more for alternatives, like Newt Gingrich, who was rising but still low in the polls.
In summary, October was the month in which Romney--lucky so far--saw his path to the nomination open completely. Since voters will become even more focused on electability as Iowa approaches, and since there are no conservative alternatives who are highly electable, Romney has likely triumphed. The nomination is his to lose. Elephant Watcher calculates that Romney's odds of winning the Republican nomination are currently 65%.