Democratic Sen. Bernie Sanders is a slight favorite heading into tonight's #IowaCaucuses. But four different candidates have led the polls over the past year, and the campaign in Iowa has been marked by uncertainty for months.

Democratic Sen. Bernie Sanders is a slight favorite heading into tonight's #IowaCaucuses. But four different candidates have led the polls over the past year, and the campaign in Iowa has been marked by uncertainty for months. https://t.co/XR7yh23xZI

— NPR Politics (@nprpolitics) February 3, 2020