Peggy Noonan touched a little on this topic, today - in a way:

Domestically, the Democratic presidential candidates appear only before supportive groups. They don’t speak to antitax groups and talk about their own assumptions regarding tax policy. They don’t go to traditional values groups.

...it’s unworthy of a great nation. When people say the campaign feels artificial, that’s what they mean. It’s not John Edwards’s hairspray or Hillary Clinton’s makeup. It’s that they give every sign of being afraid to speak and listen to those who haven’t been patted down by thought-cops for unacceptable views.

The Republicans are the same. An invitation to debate on Univision, the Spanish-language network? They have scheduling conflicts. What about the Log Cabin Republicans? No time right now. How unserious.

Our politicians are pandering and over-careful-to-a-fault partly because what Meyer noticed about the extremists in each party, who have intimidated their candidates, monetarily and otherwise, into a droning elusiveness. No one wants to be pinned down as standing for much of anything. Candidates bob and weave their way through debates and interviews hoping nothing sticks. And they talk before vetted, friendly audiences because they don’t want the boos to end up on YouTube; most candidates will not have Viacom editing out the boos for them.