This paper deals with the electoral and political consequences of urban region formation. The electoral geography of new political parties differs substantially from that of traditional ones. New parties are mainly successful in different parts of urban regions. The declining traditional parties have rather a regional pattern, although some of them show new spatial patterns too. These developments are interpreted in the context of the cleavage theory, in which old and new cleavages are linked with a different spatiality. Following the Anglo-Saxon literature an increasing process of polarisation is hypothesised between the welfare state orientated city and a neo-conservative and neo-liberal suburban fringe. This article examines and proofs the existence of these processes in the urban region of Brussels by means of individual-level and ecological electoral data.

Parties supposedly take great care to assure an equilibrium in the socio-geographical composition of their lists of candidates. In this contribution it is investigated whether the parties which participated to the 2004 regional elections in Brussels indeed presented lists in which the criterion of equilibrated geographical distribution was taken into account. We compared the presence of candidates living in disfavoured neighbourhoods, middle class neighbourhoods and rich neighbourhoods for the different lists which participated in the elections. It is shown that elected politicians overwhelming tend to live in the richer areas of the Brussels Capital Region. This is not due to a funnel effect in which predominantly the candidates living in richer neighbourhoods were able to get elected. Parties had, indeed, in general distinct socio-geographical profiles of their candidates and these merely got reflected in the overall results of those getting elected.

The relationship in political terms between the Brussels Capital Region and the Brussels communes is characterized by the cumulation of mandates. On average 68% of the members of the Brussels Regional Parliament simultaneously held an office in one of the 19 communes. At the regional elections of 2004 even twelve of the 19 mayors were directly elected into the Brussels Parliament. This feature of intertwining mandates was mainly valid for the French liberal party, the FDF and the Frenchspeaking Christian-Democrates. In this manner not only did the Brussels communes have access to the Parliament but to the Brussels Regional Executive as well. The cumulation of mandates is though but one example of the intertwining of the Brussels Capital Region and its communes.

At least four criterions/methods to measure mechanical effects of electoral systems can be distinguished: measuring disproportionality, the reduction in number of parties, the party advantages and the threshold percentages. In this manuscript we focus on the thresholds. We first concentrate on a description of legal, theoretical, and empirical thresholds as measures of mechanical effects. Further, we analyse the relationship between (the natural logarithm) of district magnitude and the empirical threshold and between the empirical threshold and the effective number of parties. As starting point we take districts in Spain, Portugal and Hungary as the level of analysis. We clearly show that there is a negative causal connection between district magnitude and the threshold percentage and between threshold percentage and the number of parties.