What drives candidates to "go negative" and against which opponents? Using a unique dataset consisting of all inter-candidate tweets by the 17 Republican presidential candidates in the 2016 primaries, we assess predictors of negative aff ect online. Twitter is a free platform, and candidates therefore face no resource limitations when using it; this makes Twitter a wellspring of information about campaign messaging, given a level playing-fi eld. Moreover, Twitter's 140-character limit acts as a liberating constraint, leading candidates to issue sound bites ready for potential distribution not only online, but also through conventional media, as tweets become news. We fi nd tweet negativity and overall rate of tweeting increases as the campaign season progresses. Unsurprisingly, the front-runner and eventual nominee, Donald Trump, sends and receives the most negative tweets and is more likely than his opponents to strike out against even those opponents who are polling poorly. However, candidates overwhelmingly "punch upwards" against those ahead of them in the polls, and this pattern goes beyond attacks against those near the top. Sixty of 136 dyads are characterized by lopsided negativity in one direction and only one of these 60 involves a clearly higher status candidate on the off ensive.