The political party can be both a source of institutional and personal power for a prime minister. Yet parties can also constrain and ultimately remove prime ministers. This chapter compares the experience of prime ministers as party leaders in Britain and Australia under the tenures of Tony Blair and John Howard. It considers how this relationship was defined and changed over time. Predominant prime ministers such as Blair and Howard cultivated greater internal power over the party and greater autonomy from the party. Yet, such leaders are still susceptible to, albeit limited, parliamentary party pressure and predominance can accelerate the 'hollowing out' of their parties.