Rattachisme is a part of the Walloon movement

Walloon Movement

The Walloon Movement is an umbrella term for all Belgian political movements that either assert the existence of a Walloon identity or defend French culture and language within Belgium. The movement began as a defence of the primacy of French but later gained political and socio-economic objectives...

 that advocates the unification of Wallonia (and maybe Brussels

Brussels

Brussels , officially the Brussels Region or Brussels-Capital Region , is the capital of Belgium and the de facto capital of the European Union...

 and the six Flemish municipalities with language facilities for French-speakers) with France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

.

The main rattachist political parties are the Walloon Rally

Walloon Rally

The Walloon Rally is a Belgian political party, active in Wallonia since 1968. The party favoured federalism and since 1985 independence....

 (partially) and the Wallonia-France Rally. They still have not gained any parliament seats so far.