From the Pew Research Center, which reports:
Asked in an open-ended format, 33% of Trump supporters and about as many Clinton supporters (32%) frame their vote at least partially in opposition to the other candidate, often using harsh language.
Among Clinton supporters, only her experience, at 32%, is mentioned as frequently as opposition to Trump. For Trump supporters, opposition to Clinton is among the most frequently cited factors for supporting their candidate, with nearly as many citing his status as a political outsider (27%) or his policy stances (26%).
This “opposition as support” takes place in the context of an election campaign that is far more likely to be viewed in negative than positive terms: Majorities of Americans describe themselves as “frustrated” and “disgusted” with the campaign, while few declare themselves “interested”, “optimistic” or “excited.” And these negative takes have only become more widespread over the course of the summer.