Marco Rubio, R-Fla., speaks at a town hall meeting Thursday in Cedar Falls, Iowa. Charlie Neibergall/AP hide caption

itoggle caption Charlie Neibergall/AP

Nobody really knows whether Vice President Biden will run, but there's a debate in Democratic circles about whether he can afford to wait. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Trump speaks to an audience during his trip to the U.S.-Mexico border in July. He has called for a wall on the border and mass deportation. Matthew Busch/Getty Images hide caption

itoggle caption Matthew Busch/Getty Images