We exploit the act of the conservative Greek government (2004–2009) to fiddle the books as a natural experiment in order to document a causal link between government spending and electoral fragmentation and identify the mechanism via which it operates. The retrospective revision of Greece’s deficit figures just before the 2010 regional elections constituted an information shock, which generated expectations for reduced pork-barrel spending. We decompose the resulting effect and uncover the main mechanism taking place: rent-seeking voting and patronage (client-voters abandoning the dominant parties due to less expected rents). We find that expected spending cuts caused a steep decline (increase) in the electoral support for dominant parties (fragmentation). This effect is significantly more pronounced in patronage-intense regions. Using the size of public sector as a proxy for patronage (Hicken 2011), we find that support for dominant parties declined differentially by 5 percentage points more on those regions. That is, at least one in six voters that abandoned the big parties did so out of purely opportunistic motivations. Overall, our work highlights the importance of institutional constraints in affecting electoral and political power-sharing.