Operating within these regimes, Obama was able to mount a credible--and ultimately successful--challenge to the leadership's choice for the nomination while Lopez Torres could not. This article offers an explanation why. It argues that Obama succeeded where Lopez Torres failed because the nomination process Obama traversed was more penetrable and more contestable than the one Lopez Torres faced.
Katz, Ellen D. "Barack Obama, Margarita Lopez Torres, and the Path to Nomination." Election L. J. 8, no. 4 (2009): 369-81.