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I think your comments on the failure of liberal talk radio overlook one key factor.
Talk Radio is driven by an endless discussion of issues. There are about 50 issues that the right tend to (more or less) agree on -- guns, taxes, gay marriage, prayer in school, affirmative action, military, and down the line. So, a relatively large base can tune in at any time and hear something that jibes with their core beliefs.
The left is much more factionalized. Do the greens really care about inner-city issues? Do pro-union democrats want to hear about gun control and gay marriage? Do pro-abortion feminists all have the same take on NAFTA? How do Democrat farmers feel about "drill here, drill now?"
I contend that voters on the right tend to embrace the party line on 80% of the issues, while any given democrat probably doesn't care about 60% of their party's issues. And they won't tune in to a rant about which they do not care or disagree.
Simply compare the number of bills that Gingrich was able to pass with a slim majority in Congress to the impotence of Pelosi with a similar majority. The right is unified in their views; the left is all over the board. And that, in my view, is why Liberal Radio can't hold an audience.