In Belgium, there is the Flemish Movement. Belgium is split in two areas, very roughly speaking it is Flanders in the North and Wallonia in the South. Wallonia is the economically poorer part nowadays, but the Flemish Movement already started when this was not the case and when Flanders was over-ruled by the rich South:
In the 19th century, the area began to industrialize, mainly along the
so-called sillon industriel. It was the first fully industrialized
area in continental Europe, and Wallonia was the second industrial
power in the world, in proportion to its population and its territory,
after the United Kingdom.
Wallonia is rich in iron and coal, and these resources and related
industries have played an important role in its history.
Nowadays, after the decay of the mining industry, Wallonia has become the weaker part of the economy, and the old resentments are still there: in addition to the new costs of the South.
In recent history, the Flemish Movement has increasingly grown amid
the 2007-11 Belgian political crisis and its aftermath. Since
2010, the separatist N-VA party has been the biggest polled in
Flanders, while Vlaams Belang, has become the second largest in the
2019 federal and regional elections.
In the last decade, Belgium was at the threshold of partition. Still, partition of Belgium has its own wikipedia entry more for historical reasons. That is why it does not fully count as an answer here: since this movement has already grown the most when Flanders was the underperforming part, and the underperformance of Wallonia is likely not the main reason for the power of the separatist N-VA party or 2019 Vlaams Belang, but history. The clearest reason seems to be written in these lines about the start of the Flemish Movement:
French was the only official language of Belgium until 1898, even
though Flanders was and still is predominantly Dutch-speaking. The
government's long refusal to acknowledge Dutch as an official language
led to hostilities between Flanders and the French-speaking
bourgeoisie who held both political and economic power. These
hostilities gave rise to the Flemish movement, which began as a
literary and cultural organization, but later became a political
movement that called for legal recognition of Dutch and for social
emancipation of the Flemish people.
That Flemish movement was obviously a reaction to the Walloon Movement starting in 1880:
The Walloon movement arose in the 19th century along with the language
disputes; French-speakers sought the preservation of the French
language and culture as the defining creed of the country.
Of course, the Walloon economic "underperformance" of today will still play a role, therefore this post, but it does not fully fit.