Early polling leaders often win primaries

Average support in polls taken in the first half of the year before presidential primaries (1972–2016) for candidates with high name recognition and their chance of winning the nomination

Name recognition is based off a five-tier scale in which candidates are classified as having 20, 40, 60, 80 or 100 percent name recognition based on polling averages and some subjective adjustments. "High name recognition" is candidates who had 60, 80 or 100 percent name recognition.
FiveThirtyEight
Source: Polls