I've said this before and I'll say it again, there's a fallacy that their is this mythical independent vote that lays in the very center of the spectrum between both parties. They think the goal is to shore up the base quickly so you can run right back to the center and fight that fight.
What is lost in this is that the Democratic establishment and the Republican establishment have monopolized that center area. If you are a moderate or a centrist you already have an abundance of choice between the two parties. You have the Nancy Pelosi's and Clinton's on one end and the Romney's and Ryan's on the other. The country is skewed so obviously that is far apart, but there really aren't that many between those two factions that are going to have a tough time finding a party to stick with.
Yet there are still independents and young people who don't turn out. They seem to really congregate around candidates who are to the far left or far right. More so the libertarian faction on the right (the Paul far right, not the Cruz far right) and the Democratic socialist far left.