Sen. Bernie Sanders (D-VT)

With Democrats facing tough prospects in the midterm elections, will they also have to worry about primary challenges from their left flank?

During his presidential campaign, Sen. Bernie Sanders never quite decided whether his "political revolution" could coexist with the Democratic Party or needed to overthrow it. Now, Sanders is one of the most popular political figures in the country and the activists he galvanized have gone on to start new groups, join old ones, or run electoral campaigns.