A clear top tier has emerged in this month's Iowa Caucus poll

The latest Iowa State University/Civiqs poll of Democratic presidential candidates finds the same 4 people receiving the most support in Iowa. Mayor Pete Buttigieg continues to lead this month, with Senators Bernie Sanders and Elizabeth Warren right behind him. Former Vice President Joe Biden moved up a few percent. Iowa State political science professor Dave Peterson talks about the poll's steadiness to last month, and the difficulty the rest of the candidates have to move up.