When it comes to federal elections, third party candidates are almost assured a defeat. And yet the Libertarian Party, the Green Party, the Reform Party -- these underdogs always appear on the scene ready for a fight. So why run if you're not going to win? What do third parties do to American politics? Our mediators for this one are Marjorie Hershey, Professor of Political Science Emerita at Indiana University and Geoffrey Skelley, Elections Analyst at FiveThirtyEight.