Constitutional reasoning falls mainly under the description of political reasoning in that many of its dispositions admit of different conceptions and interpretations. The question, however, is whether any token of constitutional reasoning as such is political by definition. This issue is particularly relevant when it comes to the constitutional guarantee against ex-post-facto criminal legislation. Although this guarantee used to sway legal thought uncontestedly for ages, it has been recently challenged by scholars and judges, at the very least in Argentina. In this work I would like to put forward two hypotheses that explains this recent resistance: a comeback of classical penal republicanism and the hegemony of interpretative jurisprudence.