That's quite a collapse for Bachmann. During the summer, she was in first place in most Iowa polls. After Perry entered the race, she was still running second for a few weeks. Elephant Watcher has always voiced skepticism concerning Bachmann's campaign: She has never been given above a three percent chance of winning the nomination, and she is currently rated at 1%. Still, what happened to Bachmann?
Bachmann's central liability is the fact that she is considered unelectable. She has never won state-wide office and is prone to gaffes. Her reputation among independent voters is that of an extremist or a laughing stock. Her central asset is the purity of her conservative record. As for rhetorical skill, she is weak. What this adds up to is a candidate who can be easily cast aside and replaced if another conservative comes along who is either electable or rhetorically gifted.
Perry greatly assisted in the downfall of Bachmann. Before the attacks on his conservative credentials began in the September debates, he was viewed as an electable alternative to Bachmann. As soon as Perry entered the race in August, Bachmann's supporters immediately jumped ship. During September, Perry began to fall due to attacks against his conservatism and electability. But the former Bachmann supporters didn't jump back. By that point, their attention was already shifting to Herman Cain.
Cain, like Bachmann, is considered unelectable. But unlike Bachmann, Cain possesses charisma. Among the original Bachmann supporters, some wanted a more electable candidate, and they went to Perry. The rest, who demanded ideological purity, went to Cain. Since no one had yet attacked Cain's conservatism, voters assumed Cain was simply a charismatic version of Bachmann.
Meanwhile, perhaps a slice of Bachmann's former support went over to Newt Gingrich. Since he is neither viewed as ideologically pure or highly electable, we might suppose that he simply represents another "not Mitt Romney" figure. At the same time, voters experiencing an early "strategic shift" (putting the desire to defeat Barack Obama above all else) went to Mitt Romney.
If you want someone ideologically pure, you go to charismatic Cain (until someone attacks his conservatism, that is). If you want someone mostly conservative with more credentials than Bachmann, you go to Perry. If you want someone who can beat Obama, you go to Romney. If you're too disgusted by Romney's flip-flops to go to him (yet), you go to Gingrich. Bachmann has thus been squeezed out entirely.