Nevers, a medium-sized city of 33,000 inhabitants in the center of France, has long been regarded as a laboratory for municipal socialism as well as for the union of the left. The socialist party has dominated the local political scene for more than four decades. In 2014, however, a list without any party label put an end to this hegemony by winning the election. The 2020 municipal election confirmed the changeover: the outgoing list won the election, this time in the first round. How to explain this lasting change in a city that has long been a socialist city, in favor of an "apolitical" list that does not benefit from partisan resources? If this change is part of a more general movement, characterized by the decline of the “Parti socialiste” (PS) and the “Parti communiste Français” (PCF), we will show that it is also rooted in the socio-demographic transformations that the city has undergone in recent decades. Such as many medium-sized French cities, Nevers is indeed confronted with a long-standing phenomenon of urban decline, particularly marked in the central districts. The departure of managers and middle-level professions and the arrival of precarious populations has contributed to the transformation of the electorate, while the theme of decline has become one of the main issues of the last elections. This article thus proposes to examine the evolution of the local political field in the light of the social, economic, and demographic transformations of the territory. To do so, several databases are used in order to quantify and map these transformations: the results of municipal elections since 1971 and census data at municipal level since 1968 and at the neighborhood level (Iris) since 1990. These data will be put into perspective with archives relating to municipal elections from 1971 to 2020 as well as interviews and observations conducted during the 2014 and 2020 campaigns. In conclusion, the urban decline, in the case of Nevers, has contributed to bring down municipal socialism in two ways: 1) by weakening its electoral base, 2) by imposing a political agenda that the socialist municipal team cannot keep, which will be blamed on them in 2014.