Irish Republicans and the Basque Nationalist Left have been close allies throughout conflict and peace. One previous commonality was that their left-wing political wings Sinn Féin and Batasuna refused to participate in the British or Spanish parliament. For over a century, Sinn Féin has abstained from Westminster. In contrast, a new Basque left group Euskal Herria Bildu (EH Bildu) decided to consistently take their seats in Madrid. Utilising various sources including interviews with Irish Republicans and left-wing Basque Nationalists, we explore why the Basque Nationalist Left has abandoned abstentionism, in contrast to Sinn Féin’s position towards Westminster. Ultimately, different historical and contemporary factors produce contrasting strategies. These include the fact that Sinn Féin secured the right to Irish self-determination while Basques have not, and public opinion. This study provides a unique comparison on why some nationalist parties abstain from political institutions if elected while others end that tactic.