Sanctions and homogeneity of intra-party preferences are the two main pathways to party unity in roll-call votes. However, only a few works have managed to properly measure the degree of polarization within the party, and therefore the link between ideological preferences and parliamentary voting behaviour has not yet been fully tested. Looking at the internal debates held during party congresses and analysing motions presented by party factions through quantitative text analysis, the present article provides a new measure of intra-party polarization that is exogenous to the parliamentary arena. This measure is used to disentangle the effect of ideological heterogeneity on MPs voting behaviour, net of the party whip. Our results show that factional heterogeneity negatively affects party unity. This effect, however, is conditional on the strength of whipping resources available to the party leader. When the electoral system or the intra-party candidate selection process allows strong discipline to be enforced, the negative effect of heterogeneous preferences on party unity is lower or no longer significant. However, since absences can be a strategy by which to express dissent while avoiding sanctions, they should be considered as an additional voting option and this is crucial to understanding the impact of intra-party heterogeneity on party unity.