Fredreka Schouten for USA Today:

The rules on what amounts to coordination, however, are so narrowly defined that super PACs are increasingly acting as shadow campaigns — deploying staff to early voting states, financing feel-good biographical ads to introduce the candidates to voters and responding to attacks from rivals.

Super PACs cannot supplant a campaign entirely. Two Republican contenders, former Texas governor Rick Perry and Wisconsin Gov. Scott Walker, each had the support of well-funded outside groups but dropped out of the presidential contest recently as they struggled to win voter support and secure the donations they needed to fund their own staff, travel, rent and other expenses, such as ballot-access fees.