Chait argues that Citizens United allowed the Gingrich campaign to flourish:
Money is the primary mechanism that parties use to herd voters toward the choices the elites would prefer them to make. The nomination of George W. Bush offers a classic example. Bush and his network had organized so many Republicans to donate so much money that the contest was essentially over well before a vote had been cast. The Bush fund-raising network didn’t involve a handful of billionaires in a room. It required thousands of fairly affluent people working together.
In contrast, Gingrich's Super PAC, funded largely by one very wealthy supporter, is keeping him competitive moneywise.
The Dish 