In parliamentary systems, Governments play a major legislative role: they draft and propose legislation, and build the necessary coalition to ensure that a given bill will become law. Other parliamentary groups are often seen as residual, having very little influence on the policy making process. However, some parliamentary groups might be more effective than others at inserting their policy proposals into enacted legislation. Due to data and methodological limitations, large-N and systematic studies of the conditions under which parliamentary groups are successful at influencing legislation are extremely rare, leaving crucial questions about who and why has power at influencing legislation unanswered. To what extent do parliaments reverse executive proposals? What parliamentary groups are more efficient in performing this task? In order to answer these questions, first, we compare executive bills proposals as they enter the legislative process with the final text of laws using text reuse methods. Next, we identify the parliamentary group responsible for the different amendments and calculate the amendments’ efficiency rate for each group. The theoretical framework explores causality taking into consideration the macro level (such as the characteristics of bills, the type of government or the electoral cycle), but also the micro level (parliamentary groups’ resources, including past parliamentary experience, specialization and expertise, and ideological distance from the incumbent). The analysis relies on a new database containing all the executive bills introduced into the Spanish Parliament from 1996 to present.