In several countries, research shows that mainstream news media have increasingly become a target of political attacks, and a recurring accusation is that the news media are biased politically. Since social media play an important role in circulating accusations of media bias, in this paper we analyze 167,527 Swedish tweets about media bias over a ten-year period using the mixed-methods CADS approach, which incorporates computational processes with discourse analysis. Specifically, we explore the prevalence of bias accusations over time and which news outlets constitute the main targets; from what political perspectives these bias accusations have originated over time; and how discourses accusing the news media of political bias are constructed. Among other things, the results show that accusations of left-wing bias are much more common than accusations of right-wing bias. A key reason is that actors on the political right engage much more frequently in accusations of media bias than their center-left counterparts, and that discourses of media bias are heavily dominated by right-wing populist tropes in which mainstream media are part of the "corrupt", lying elite who spread "politically correct" disinformation.