This paper proposes a methodology for investigating populism on social media by analyzing the emergence of proto-slogans, defined as nominal utterances (NUs) typical of a political community on social media. We extracted more than 700.000 comments from the public Facebook pages of two Italian populist parties' leaders (Matteo Salvini and Luigi Di Maio) during the week preceding the 2019 European elections (i.e., from May 20 to May 26, 2019). These comments have been automatically clustered and manually annotated to find protoslogans created by the parties' supporters. Our manual annotation consists of four layers, namely:Nominal Utterances (NUs), a syntactic device widely used for slogans; Slogans for NUs with a slogan function; Top-down/Bottom-up, to recognize the slogans produced by the politicians and those produced by supporters; Proto-slogans, for NUs devoid of specific political content that nonetheless express partisanship and support for the leaders.