Using a novel statistical method, we estimate the vote counts of the secret elections held by the National Congress of the Chinese Communist Party from 1945 to 2017. We also construct a metric for voter preference diversity based on a standard index of legislative party fractionalization. We find that both the number of dissenting votes and the voter preference diversity index plummeted to the bottom in 2017, which is an unprecedented phenomenon in the party's history. Applying the same method to provincial party congress elections from 2006-2017, we find that provincial dissenting votes also declined around the same time, though the magnitude is relatively smaller and there is a wide range of provincial variations. We suggest that President Xi's suppression of formal party institutions is complete at the national level but not yet at the provincial level.