Former Vice President Joe Biden (left) and Sen. Bernie Sanders lead the crowded Democratic presidential primary field, according to the latest NPR/PBS NewsHour/Marist Poll. Above, they appear during the Democratic presidential primary debate in Atlanta on Nov. 20.

Former Vice President Joe Biden (left) and Sen. Bernie Sanders lead the crowded Democratic presidential primary field, according to the latest NPR/PBS NewsHour/Marist Poll. Above, they appear during the Democratic presidential primary debate in Atlanta on Nov. 20.