As political toxicity has become widespread on social media, scholars have sought to understand what drives it. While studies often emphasize platform-level explanations, such as algorithmic amplification, they offer limited insight into which users are more likely to produce toxic content. Even less is known about how the relationship between individual-level factors and toxicity varies across political contexts. Using data from 184,354 randomly sampled U.S. Twitter (now X) users, their original tweets (25M), and the accounts they follow, we link users’ inferred characteristics and political context to their real-life toxic behavior. Results show that Republican, male, and older users are consistently more likely to post both impolite and intolerant content, whereas education shows no clear association. We also find that state-level electoral competitiveness increases overall toxicity and, in some cases, conditions individual-level relationships. The age gap in impolite behavior significantly narrows in more competitive states, while the corresponding moderation for intolerance is only marginally significant. There is also some indication that partisan differences in impoliteness become larger in more competitive states, but this moderation is only marginally significant. Together, these results demonstrate that political toxicity is shaped jointly by users’ individual characteristics and the political contexts in which they operate.