Poll shows 2 man race. But why?

A new USA today / Gallup national poll shows Former House Speaker Newt Gingrich as the front-runner to be the GOP nominee with 22% of the vote, but Former Governor Mitt Romney (R-MA) polls just behind him at 21%. The only other candidate in double-digits is Businessman Herman Cain, who polls at 16%. Up until the media began spreading the story about Herman Cain's alleged sexual misconduct, Cain was Romney's challenger for the nomination while it seemed Gingrich would have a fighting chance. Now, the roles of Cain and Gingrich have reversed. But why? Part of this, in my view, is a 5 way divide within the republican party between the establishment, the Reagan coalition which is more conservative, the moderate-wing of the party, the grassroots, and the tea party. The establishment is really a small group of well-funded individuals and elected members of congress. Of course, this group favors Romney, which is why Mitt Romney has stacked up so many congressional endorsements. The moderate wing of the party, who has not had a nominee since George H.W. Bush, also supports Romney, though I'm sure a few are looking at Jon Huntsman at least in New Hampshire. The Reagan-wing of the party, the conservatives who make up a large majority of the party, are likely divided between Mr. Gingrich and Mr. Romney. Grassroots activists are either supporting Ron Paul or Newt Gingrich, the conservative activists going for Gingrich while the libertarian activists are going for Paul. The tea party has 4 candidates in this race; Gingrich, Cain, Paul, and Michele Bachmann. Bachmann effectively takes votes away from Cain, where as Gingrich and Paul maintain a loyal group of supporters. The combination of Reagan conservatives, grassroots conservatives, and tea party voters help Gingrich challenge Romney, who is backed by the party establishment and moderates. Romney also has some Reagan conservatives backing him, but those Reagan conservatives are likely less conservative than those supporting Speaker Gingrich. The reason the polls are so close is because the establishment does not like Gingrich, and grassroots conservatives dislike Romney. Gingrich has sometimes appeared as overly blunt and undisciplined to the establishment, where as Romney has been seen as indecisive and too moderate for the grassroots conservatives and the tea party. Both Gingrich and Romney need to overcome one big obstacle though: winning the support of social conservatives. Three second-tier candidates may split the socially conservative vote: Rick Perry, Michele Bachmann, and Rick Santorum. Herman Cain will win some social conservative votes, but I think the economy will be the top issue, which benefits both Romney (from his business career) and Gingrich (unemployment hit as low as 4.2% when he was House Speaker.) So, this two man race is between the establishment and center-right of the party and the conservative-wing of the party which consists of Reagan republicans, tea party voters, and grassroots activists.