Public opinion and election prediction models based on social media typically aggregate, weight, and average signals from a massive number of users. Here, we analyze political attention and poll movements to identify a small number of social "sensors" — individuals whose levels of social media discussion of the major parties' candidates characterized the candidates' ups and downs over the 2016 U.S. presidential election campaign. Starting with a sample of approximately 22,000 accounts on Twitter that we linked to voter registration records, we used penalized regressions to identify a set of 19 accounts (sensors) that were predictive for the candidates’ poll numbers (5 for Hillary Clinton, 13 for Donald Trump, and 1 for both). The predictions based on the activity of these handfuls of sensors accurately tracked later movements in poll margins. Despite the regressions allowing both supportive and opposition sensors, our separate models for Trump and Clinton poll support identified sensors for Hillary Clinton who were disproportionately women and for Donald Trump who were disproportionately white. The method did not predict changes in levels of undecideds and underestimated support for Donald Trump in September 2016, where the errors were correlated with discussions of protests of police shootings.