We illuminate the dynamics of primary campaigns by looking at how individuals changed their candidate support, favorability and viability within a crucial state in the 2020 primary. Via a multi-wave survey of likely primary voters in Michigan, we show that Joe Biden’s comeback win was due primarily to the change in perception of his viability following South Carolina and Super Tuesday, especially among the more moderate portion of the Democratic electorate. In subsequent county-level analyses we find that the difference between Bernie Sanders’s win in 2016 and loss in 2020 was more a function of changes in the Democratic primary electorate than in his appeal. Our work suggests that gains in viability do not benefit all candidates equally. Moreover, viability can be generated later in the schedule than previously thought and to decisive ends. We consider the implications of our findings for understanding Michigan’s outcome in November.