Endorsement ads are primarily used by incumbents and female candidates in the early stages of campaigns.

Political endorsements play an important role in shaping public opinion about candidates. While scholars have researched the impact of endorsements, few have examined the factors that determine how and where endorsements are used in campaigns. Using political advertising data from the 2008 election cycle, Newly Paul and Chance York find that four factors—candidate characteristics, campaign characteristics, ad format, and timing—affect whether an ad containing an endorsement is aired. They find that endorsements are more likely to appear in positive ads, are primarily used by incumbents and female candidates, are less likely to appear in competitive races, and tend to air less frequently as a campaign progresses.