The far-right Vox party is on the verge of securing a share of power for the first time in Spain after a snap election in the Castile and Leon region left it the most likely kingmaker for the larger centre-right People's Party.
The PP called the election in the hope of jettisoning the previous centrist partner it ran the region with by winning an absolute majority.
However, the PP failed, securing just 31% of the vote to the 30% of the Socialists - the national ruling party - and Vox's 17%.
Vox's national chief Santiago Abascal said the party's local leader could become the deputy regional president in Castile and Leon.
"Vox has the right and the duty to form a government in Castile and Leon," he told a political rally on Sunday evening.
The result is a blow for the PP, which until a decade ago was the only conservative force in Spain but saw its vote split by a series of corruption scandals, and a boon for Vox, which was started in 2014 and is now the third largest party in parliament.
It is also another gain for the far right in Europe, which has extended support in countries such as France, Italy and Portugal by taking aim at immigration, feminism and LGBT or other minority communities.
Under Spain's quasi-federal system, the regions have their own parliaments and governments with powers over areas like Health and Education.
Vox has previously lent its votes to conservative regional governments in regions like Madrid or Andalusia on specific legislative topics, but has never before joined a regional government formally.
Vox's foothold in Castille and Leon could foreshadow another rise in support in Andalusia, Spain's most populous region, which will hold elections in the coming months, and eventually in national elections due in 2023.
Castile and Leon's current regional president, the PP's Alonso Fernandez Manueco, said he would talk to all political parties to form alliances without referring directly to Vox.