Conservative radio talker Rush Limbaugh is, without question, one of the most (probably top five) influential voices in politics and media. The closest thing liberals have to claim as their own in terms of gravitas is "The Daily Show's" Jon Stewart.
Jeffrey M. Barry and Sarah Sobieraj, authors of “The Outrage Industry: Political Opinion Media and the New Incivility," take a stab at why there's no real liberal answer to Limbaugh (via Salon):
Conservative dominance of political opinion radio, then, has both demand-side and supply-side explanations. On the demand side, conservatives’ greater distrust of mainstream media and greater interest in black and white narratives, and the niche talk alternatives for African American and Hispanic audiences help us to understand why conservative audiences might be larger than liberal audiences. Meanwhile, on the supply side, the early success of Rush Limbaugh, spurred executives — prone to build on financially successful models — to develop more programs in this vein.