This contribution considers how niche parties react when they lose their niche, using the cases of three parties in the turbulent period prior to the 2019 UK general election: the Brexit Party, the Green Party and Change UK. I overview the background of these parties before showing that each lost its respective policy niches to larger, more established parties. I show that each responded with some combination of directly competing with the mainstream party; electorally cooperating with them or other parties; or diversifying into something distinct from their mainstream analogue. I explain how each party’s approach partially explains their 2019 general election result, as well as European Parliament elections result, using British Election Study data. I suggest that this ‘compete, cooperate or diversify’ approach provides a theoretical framework for understanding how niche parties are likely to react to losing their niche elsewhere.