Former Sen. Rick Santorum, R-Pa., who was lagging in the polls in 2012 but ended up winning in Iowa, says debate criteria this time are too exclusive. Jae C. Hong/Associated Press hide caption

itoggle caption Jae C. Hong/Associated Press

Hillary Clinton delivers the keynote address at the 18th annual David N. Dinkins Leadership and Public Policy Forum at Columbia University in New York on April 29. Trevor Collens/AFP/Getty Images hide caption

itoggle caption Trevor Collens/AFP/Getty Images