With the establishment of the Nominated Assembly in July 1653, Harrison appeared to be at the height of his inuence, with some contemporaries regarding the new regime as his brainchild. Harrison had reinforced Cromwell’s providential millenarianism as they turned to the Saints to reform and establish godly rule. e subsequent failure of the Nominated Assembly has, for some, been linked to Harrison’s failings as a politician. He has been seen as not pragmatic enough to succeed, as ‘sadly wanting in the arts of political strategy’.1 A more thorough examination of Harrison’s involvement in the normal day-to-day minutiae of government and the politics of the Rump and Nominated Assembly presents a more nuanced picture of Harrison as a parliamentarian as well as providing more detail of his interests and political network. Harrison’s position in and attendance on the Councils of State of the Rump and Nominated Assembly indicates a record that varied over time and suggests a uctuating engagement with one of the inner circles of inuence of both regimes, and helps explain why he moved against the rst and disengaged with the latter. is thus establishes a more detailed context from which to judge his role in the failure of the Nominated Assembly. Examples of Harrison’s political engagement in the Rump Parliament show that while for the majority of the time he functioned as other MPs, his prominent role in the expulsion of two fellow MPs also marks him as a radical driven by his religion. Furthermore, Harrison’s heavy involvement in parliamentary committees and his voting record as a teller both help illustrate his relative activism and radicalism, as well as hinting at the decline in his relationship with the traditional parliamentary political process. is statistical evidence is also set alongside the context provided by Harrison’s role in relation to key political issues that caused tension in the Rump: the Dutch War, the settlement of Ireland, the removal of tithes and the reform of the law. An examination of the changing nature of Harrison’s working relationship with the leading republican Rumper MP, Arthur Haselrig, provides another layer to help interpret why and when Harrison came to be a leading gure in the military coup that removed the Rump in April 1653.