Polling in presidential primary campaigns is notoriously fluid. In an effort to gauge movement among voters as they might change their mind and as candidates pop in and out of the race, pollsters will often ask voters about both their first-choice candidate and their backup. This question has been particularly popular this cycle, given how crowded the field is, and can be useful for understanding the overall level of support a candidate has, including who benefits if one candidate loses support or whether voters are grouping candidates into “lanes.” CONT.
Dave Peterson (Iowa State U.), FiveThirtyEight