Parties usually argue in favour or against a government coalition based on party considerations in terms of projected policy implementation, power in office and vote maximization – that is, the ‘policy, office, votes’ triad. So far, however, it remains unclear which claims mainstream parties invoke to motivate their choice to rule or not rule with populist parties. Adopting the ‘policy, voter, office’ triad, this article examines mainstream parties' Twitter claims on ruling with populist parties in Austria, Belgium, Germany and the Netherlands (2006–2021, N = 1,919). Mainstream parties mainly reject ruling with (mostly radical right) populist parties. To justify unwillingness, policy-based motives referring to the populist parties' extremist nature trump motives on office-seeking and vote maximization. To justify willingness, predominantly office-seeking motivations are invoked. Party characteristics (ideology, incumbency status, size) and context, however, shape these claims. This study sheds light on mainstream parties' patterns of political communication on coalition formation with populist parties.