When appealing to a political audience as broad as the voting public, you are confronting a large majority of voters who pay relatively little attention and are essentially non-ideological in their political orientation. That means the idea of converting somebody from "liberalism" to "conservatism" as a precursor to getting his vote is simply not going to yield many votes. If it did, this is what candidates - who have the greatest interest in winning votes - would try to do. Instead, they speak in sound bytes and they have Stevie Wonder or Hank Williams, Jr. open their political rallies.
But it's precisely because symbolism matters as much or more than substance that the amount of attention Rush Limbaugh is grabbing at the moment is bad for the GOP. Yes, elections aren't won by having a bunch of clever conservative intellectuals go around writing columns and giving speeches that convert people from liberalism to conservatism. And they aren't won, usually, by clever policy proposals either. But they are won, quite frequently, by politicians and parties that present themselves as the sort of people who seem to be interested in clever policy proposals, and who seem as well like they might be able to convert you to their way of thinking (and be interested in hearing about yours) if they had a few hours to talk to you about the matter. And the success of Barack Obama - his ability to woo non-ideological and politically underinformed voters - flowed and flows in part from his ability to project just these qualities, even as he pushes a substantive agenda that's left-liberal to the core.
Rush Limbaugh, on the other hand, projects a different set of qualities, and comes freighted with a different set of associations. Rich Lowry, while disagreeing with the thrust of Limbaugh's critique of would-be conservative reformers, says he finds "the attacks on Rush from the right mostly stupid, cringe-inducing, and wrong," citing as a prime example this David Frum post, which references Limbaugh's weight, history of drug problems, and various other personal foibles while drawing a contrast between a liberalism embodied by Obama and a conservatism embodied by El Rushbo. Now maybe Frum's decision to get personal is cringe-inducing, his overall point is neither wrong nor stupid: To a non-ideological voter who's uninterested in policy and forms his perceptions of liberalism and conservatism largely through symbolism and sound bites, a conflict between Obama on the one hand and Limbaugh on the other will almost inevitably redound to liberalism's benefit.
This doesn't mean that the Limbaugh v. Obama dynamic that Rush and the Democrats are mutually laboring to cultivate is going to be decisive in future elections and debates. I certainly wouldn't disagree with Cost's point that "elections are fought over the state of the union and the country's opinion on how the majority party has managed the government," and that parties and politicians mainly get to tinker at the margins. But a lot of action happens there nonetheless - and the fact that it isn't going to determine the outcome of the 2012 election doesn't change the fact that the Limbaugh-related action over the last couple of weeks has been bad for conservatism's image, and its political prospects as well.