Supporters of Sen. Bernie Sanders cheer during a protest near City Hall in Philadelphia during the Democratic National Convention in July. John Minchillo/AP hide caption

Democratic presidential candidate Hillary Clinton (left) and Sen. Elizabeth Warren wave to the crowd before a campaign rally at the Cincinnati Museum Center at Union Terminal on Monday. John Sommers II/Getty Images hide caption

Democratic congressional candidate Lucy Flores speaks at a campaign rally for Democratic presidential candidate Bernie Sanders in February in Las Vegas. Ethan Miller/Getty Images hide caption

Democratic presidential hopeful Bernie Sanders speaks to supporters at an election rally in Ventura, Calif., this week. Mark Ralston/AFP/Getty Images hide caption

Guests stand after a Rand Paul speech at the Conservative Political Action Conference (CPAC) in Rosemont, Ill. Conservatives celebrated Tuesday's victory in Wisconsin's recall election. Scott Olson/Getty Images hide caption