Seen as the best form of democracy, adopted in almost every country in the last century, constitutional liberal democracy is facing over the last decades a turbulent period of constitutional degradation. This paper will analyse this undergoing process, initially, focusing the attention on populism, considered as an essential element of this process when ongoing, and as a warning signal for the stability of the other democracies when the process has not started yet. It will, then, scrutinize the main institutional and legal instruments adopted by populist leaders to undermine the fundaments of a constitutional liberal democracy (Constitutional amendments, alteration of the electoral competition, weakening of the checks and balances’ system, reduction of fundamental rights and freedoms protection). Therefore, it will examine the practical application of this constitutional erosion in Hungary, Poland, Turkey and Russia, giving, thus, a comparative overview of the phenomenon. Finally, it will briefly attempt to propose some suggestions to contrast this pernicious process and protect constitutional liberal democracies.