The virtual disappearance of moderate and cross-pressured members from the US Congress is analysed in this article. There were substantial numbers of these partisan non-conformists in both parties and in both chambers until the early 1980s when the middle began to shrink. This trend continued and accelerated in the 1990s. Partisan non-conformists disappeared through
replacementand conversion. When moderate and cross-pressured members left Congress, their replacements were much more likely to be mainstream partisans in the 1980s and 1990s than they had been in earlier decades. The occurrence of some type of conversion (a shift towards the party's ideological mainstream or a party switch) is also much more common in recent decades. We present evidence that the shrinking middle in Congress resulted from electoral changes.