Representative democracy is party democracy. Parties vote together in the legislative arena; party labels act as information shortcuts in the electoral arena, tying together co-partisan legislators’ re-election prospects. But the utility of party labels is weakened by waning party identifications in the electorate. Partisan dealignment therefore risks undercutting party loyalty on the part of backbenchers. Combining district-level data on electoral volatility and new data from the PARTIREP survey of legislators in 15 advanced industrial democracies, it is demonstrated that party loyalty is lowest where partisan dealignment is strongest – even after accounting for backbenchers’ policy preferences, whether they represent the ruling party or the opposition, and their campaign strategies. Our results have important implications for the sustainability of current models of representative democracy.