Islamist political parties in the Middle East and North Africa (MENA) region have often faced inordinately challenging situations, marked by strong and concerted coercion, co-optation and persecution from regimes and often with active support at a popular level. They have also faced strong internal stresses especially due to trying to cope with the demanding external context. One would expect that this would lead to a breakdown in the organisational integrity of these parties, an expectation that is sharpened because of many examples of parties in the region where this did occur. Yet there are also some MENA Islamist parties that did manage to maintain organisational integrity despite facing such a range of extreme pressures. The question is, how have they done so? The study answers this question with a qualitative case study looking at the Tunisian political party Ennahdha during the period from 1981 to 2016. Ennahdha experienced extended periods of heavy persecution including exile, short periods of semi-legalisation though with ongoing surveillance and harassment, and then legalisation in 2011, winning national elections, leading the writing of a new constitution, stepping down from governing under strong external pressure and then becoming a junior partner in a new government. These pressures caused extensive turmoil and intra-party tensions for the party – and yet it retained organisational integrity. The study explains this by looking in detail at four factors taken from the political parties literature but not previously applied to a MENA Islamist political party case or to the case of a party experiencing such extreme and shifting pressures: religious belief as constitutive for the party’s identity, rooting of the party in a wider social movement, the effectiveness of the party’s leadership including its continuity, and the nature of structural channels for member involvement. The study shows that the factors engendered institutionalisation and provided other benefits to the party that supported its ongoing organisational integrity. It also finds that the social movement played a more complicated and less purely positive role than expected, adds considerably to understandings of how shared religious values operate in practice within the party including in discussions, decision-making and supporting ongoing commitment, and enhances understandings of intra-party conflict and conflict management as well as how Islamist parties change positions including on ideological commitments.