This chart, produced by the Wesleyan Media Project, breaks down the percentages of ad genres by political campaigns in recent election cycles from 2000 to 2012.² In 2000, around 30% of ads negative, 30% contrasted the candidates to each other, and nearly 40% were positive. By 2012, nearly 65% of ads were negative, another 20% were contrasting, and only around 15% were positive. The shift towards using attack ads to portray an opponent in a negative light began to pick up speed in the 1992 election, but this chart illustrates how much the use of negative ads continued to grow in the 21^stcentury. 

2. Ordway, Denise-Marie. “Negative Political Ads and Their Effect on Voters: Updated Collection of Research.” Journalist’s Resource, 20 Dec. 2016, journalistsresource.org/studies/politics/ads-public-opinion/negative-political-ads-effects-voters-research-roundup/.