Even though the story is about Biden, this paragraph about how the electorate, both Rs and Ds, chooses its candidates is (I feel) spot on:

"Ten years of endless blather about the game of politics on cable TV have trained the most engaged American voters to handicap candidates rather than hear them, to pontificate about who might win rather than deciding whom they actually want to win. Voters seem to approach politics increasingly as pundits, and they look to poll numbers to tell them who’s electable and who isn’t, never stopping to realize that they are the ones who get to decide."

http://thecaucus.blogs.nytimes.com/2007/12/26/joe-biden-briefly/index.html?hp