Floating signifiers are a Lacanian / Lévi-Straussian / structuralist/post-structuralist thing.

The big one is probably:

• Ernesto Laclau – On Populist Reason
  He explicitly traces how political slogans/terms become empty vessels that glue coalitions together.
  That book is basically the Wikipedia page for floating signifiers but written in academic cryptic crossword form.

Other suspects in the academic lineup:

• Claude Lévi-Strauss (original “signifier drift” guy)

• Stuart Hall (media/ideology)

• Judith Butler (performative identity language soup)

• Chantal Mouffe (politics as meaning-making chaos arena)