High party discipline and high party strength induce legislators to stay committed to the party's promised policies after their election. This is because party leaders are able to punish deviations and reward behavior that is in the party's interest. Higher political stability induces party leaders to take a longer-term perspective. We investigate the following interrelated hypotheses: (i) the effects of party discipline and party strength on environmental policy are conditional on the degree of political stability; and (ii) the effect of political stability is conditional on the levels of party discipline and party strength. Our empirical findings support these hypotheses.