About a third of American voters strongly align themselves with neither the left nor the right. So in any given national election millions votes are up for grabs. Are more of those voters going to be drawn to a calm and reassuring figure whose party has well established beliefs and values or to a candidate who does not articulate his position as well, does not instill confidence and whose most vocal followers are, well, a little on the flakey side?
George W. Bush was the candidate in this past election who presented himself as confident and fatherly, a voice of reason during a turbulent time. Even if you don't agree with his positions on relevant issues, at least you know what his positions are. He "pulled" votes from the pool of undecideds.
The Kerry camp found itself in the opposite position. And it wasn't so much the candidate himself as it was his followers who "pushed" voters away. The Hollywood left and Michael Moore are the best examples. Americans don't identify well with the Hollywood elite and Fahrenheit 9/11 probably garnered more votes for Bush than Kerry. A party that is not viewed as calm and reasonable but rather crazed and frenzied just ain't gonna get the votes. Look at how dramatically Howard Dean's popularity dropped after the "screaming incident".
So as long as the Democrat party is led by folks like Howard Dean, Harry Reid, Ted Kennedy and Nancy Pelosi, and as long as they attract rabid supporters like Michael Moore, the Republican party will only grow stronger.