On 25 October 2020, Chile voted in favour of a Constitutional Convention (or Constituent Assembly) to replace the 1980 constitution. This kind of constituent mechanism had previously been resisted by the traditional political parties, most of whom now support the mechanism. This article identifies the political groups who opposed a Constituent Assembly and argues that the shift in approach was due to three factors. First, there had been a cumulative process of social pressure and political reactions since 2006. Second, the social unrest of October 2019 raised costs for the political elite - right and left - of denying a Constituent Assembly. Third, the establishment was able to reduce the uncertainty of the constitution-making process.