About half of Democrats consistently 
Hillary Clinton led Bernie Sanders for the Democratic nomination in every Pew Research Center survey conducted throughout the party’s primaries. But many Democratic voters vacillated in their candidate support throughout this period.
Even as Clinton led throughout, the share of voters who remained consistently loyal to her over the course of the election was far smaller than her overall support in any single survey. By April 2016, near the end of the Democratic primaries, Clinton was named the first choice of 46% of Democratic voters in an open-ended question about their nomination preferences. But only 29% had consistently named her their first choice in December 2015, March 2016 and April 2016. And even fewer, just 15%, had consistently named her their first choice going all the way back to March 2015.
This fluidity wasn’t confined to Clinton supporters: Sanders was the top choice of 37% of Democratic voters in April 2016, but only about half of these supporters (20% of all Democratic voters) consistently backed Sanders across the three primary-season surveys. Fully 44% of Democratic voters changed their preferences at least once in the surveys conducted over this four-month period, including those who may have been undecided at some point.
About half of Democrats consistently  | backed a candidate through primaries  | Across three surveys, % of Democratic and Democratic  | leaning registered voters who  | Stayed undecided/Other  | 7% Consistently supported  | supported Clinton  | supported Clinton  | 29%  | Changed  | their preference  | 44%  | 20%  | Consistently  | supported  | Sanders  | Notes: Based on Democratic and Democratic-leaning registered  | voters. "Changed Sanders their preference includes those who registered named  | Clinton or Sanders in atleastone survey butdid notstay witha  | candidate throughout; includes ma nywho were undecided.  | Source: American Trends Panel surveys nywho conducted Dec 2015  | through April 2016  | PEW RESEARCH CENTER 