Will Spain remain united, or could regional strivings for more autonomy eventually tear the country apart?
Analysts seeking answers to that ever-present question will keep a close eye on Sunday's regional elections in the Basque region and Galicia.
The polls to elect 75-member regional parliaments will also help to gauge the general political atmosphere in the country hit by a deepening recession, dpa reported.
Unemployment could climb to nearly 20 per cent by 2010, but a corruption scandal affecting the conservative opposition could improve the chances of Prime Minister Jose Luis Rodriguez Zapatero's socialists.
The party is hoping for a historic victory in the Basque region and to remain part of the governing coalition in Galicia.
The two northern regions count among the ones most likely to challenge Spain's unity, along with nearby Catalonia.
The three regions have their own languages alongside Spanish, and enjoy varying degrees of self-government alongside Spain's 14 other semi-autonomous regions.
The Basque region is known for the violence of the militant separatist group ETA, which has killed more than 800 people over four decades.
The vast majority of the 2.1 million Basques oppose ETA's terrorist tactics, but around half of them traditionally vote for Basque nationalist parties which seek a wider measure of autonomy from Spain.
Sunday's poll is seen as the first in nearly three decades that could remove the Basque Nationalist Party (PNV) from power.
The PNV now faces a serious challenge from a mainstream Spanish party after the socialists increased their support in the region.
The PNV is divided internally over the radical policies of its leader, Basque regional Prime Minister Juan Jose Ibarretxe, whose attempt to organize a referendum-like vote on Basque self- determination was blocked by Spain's Constitutional Court.
Basque socialist leader Patxi Lopez, on the other hand, claims to be able to unite the Basques under the option of Spain granting the region more self-government within the framework of its current autonomy status.
A clear victory for Lopez would be a relief for Zapatero, whose attempt to negotiate with ETA collapsed in 2006.
If, however, the Basque socialists do not get an absolute majority, the ensuing power struggle could cost Zapatero the support the PNV has been giving to his minority government in the Spanish parliament.
The Basque elections will be the first ones without the participation of radical separatists after the Supreme Court barred two formations close to ETA's views from contesting them.
ETA protested the court decision by staging a car bombing in Madrid on February 9, and interfered in the electoral campaign by bombing a socialist office in the Basque town of Lazkao this week. Neither attack caused injuries.
In the Galician elections, the issue of self-determination plays a lesser role, although the Galician National Bloc (BNG) wants Spain to officially recognize "the national character of Galicia."
A coalition formed by the socialists and the BNG has governed the region of 2.8 million residents since 2005, after 16 years of rule by Manuel Fraga, a former information minister under 1939-75 dictator Francisco Franco.
Spanish conservative opposition leader Mariano Rajoy, himself a Galician, has tirelessly toured villages in an attempt to restore conservative rule despite heavy corruption allegations against his People's Party (PP) in Madrid and several other regions.
The PP tried to stave off socialist criticism by focusing on alleged interference by justice minister Mariano Fernandez Bermejo, who went on a hunting trip with a judge investigating the corruption scandal.
Zapatero finally replaced Bermejo on Monday in a move to increase socialist chances in the regional elections.
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