Sub-constituency campaigning occurs when parties focus their campaign resources on specific geographical areas within an electoral district. This behaviour was traditionally thought to occur only in single-member plurality elections, but recent research demonstrates that proportional systems with multi-member districts can also elicit sub-constituency campaigning. However, most studies of sub-constituency campaigning rely on self-reported measures of campaigning, not direct measures of campaign intensity in different regions and communities. We present novel data on geographical variations in the intensity of Flemish parties’ campaign advertising during the 2014 general elections in Belgium, which provides a direct measure of sub-constituency campaigning. Our findings show clear evidence of sub-constituency campaigning: parties campaign more intensely in municipalities where they have stronger electoral support and in municipalities with greater population density.

By examining the Belgian case, this article aims to show that methodological nationalism is strongly present in the literature on nationalization of party offer and voting behaviour. In nationalization studies, Belgium is often presented as a typical example of a denationalized country. This is true for the party offer, as it is de facto split between the two language groups since the 1980s, and therefore also voter response at the national level. However, voter response within each separate subnational party system is very homogeneous and shows interesting differences between these party systems that inform us about important electoral dynamics. We argue, on the basis of our results, that rather than stretching the concept of nationalization, it is preferable and justified to treat the concepts of nationalization of the party offer and homogenization of voter response as analytically distinct and not as two sides of the same coin.