Link is to archived Intelligencer article on study.

Archived copy of study: https://web.archive.org/web/20191219201908/http://www.omarwasow.com/Protests_on_Voting.pdf

TLDR:

Study main gist:

Examining county-level voting patterns, I find that black-led protests in which some violence occurs are associated with a statistically significant decline in Democratic vote-share in the 1964, 1968 and 1972 presidential elections. Black-led nonviolent protests, by contrast, exhibit a statistically significant positive relationship with county-level Democratic vote-share in the same period. Further, I find that in the 1968 presidential election exposure to violent protests caused a decline in Democratic vote-share. Examining counterfactual scenarios in the 1968 election, I estimate that fewer violent protests are associated with a substantially increased likelihood that the Democratic presidential nominee, Hubert Humphrey, would have beaten the Republican nominee, Richard Nixon. As African Americans were strongly identified with the Democratic party in this time period, my results suggest that, in at least some contexts, political violence by a subordinate group may contribute to a backlash among segments of the dominant group and encourage outcomes directly at odds with the preferences of the protestors.

Two-sentence summary from article:

Wasow finds that nonviolent civil-rights protests did not trigger a national backlash, but that violent protests and looting did. The physical damage inflicted upon poor urban neighborhoods by rioting does not have the compensating virtue of easing the way for more progressive policies; instead, it compounds the damage by promoting a regressive backlash.