https://phys.org/news/2018-06-negative-political-advertising.html

The study examined political advertising and its impact on the share of the vote in two-party races in 2010 and 2012 U.S. senatorial campaigns. It focused on advertising across the borders of designated marketing areas (DMAs), where discontinuities may exist that lead to different levels of exposure to political advertising. DMAs are typically used by marketers to define marketing areas by town, city or major metropolitan area.

The study authors found that negative advertising is powerful in terms of influencing preferences and voter turnout, but not across the board. When the ads are from the candidates or campaigns themselves, the negative advertising was found to be more effective. When the negative advertising was from Political Action Committees (PACs), it was not as effective.