After a tense campaign dominated by emotional issues, particularly national identity and gender equality, the likelihood that a coalition agreement will last for weeks or months will give rise to a wider sense of political insecurity across Europe.
At least five parties from across the political spectrum have a chance to be in government, and they may have difficulty agreeing on a deal between them, which means that repeating is one of several possible outcomes.
However, some things are clear, based on opinion polls and conversations with inmates of the party. No single party will get a majority; the socialist party of outgoing Prime Minister Pedro Sanchez leads the race; and there will be lawmakers from the far-right Vox party.
In addition, the result is too close for a call.
The vote begins at 9:00 am (7:00 am GMT) and ends at 8:00 pm on the Spanish mainland for the nation's third national election in four years, each of which led to a further shift in the political landscape Has.
It is not certain that Sanchez will succeed in remaining in office and how many allies he would need to collect.
If Sanchez needs the support of the Catalan separatists in addition to the left-wing anti-austerity party Podemos and other small parties, the talks will be long and the outcome unclear.
Polls that ended on Monday suggest that it will be more difficult for a right-wing split between three parties – the center-right Ciudadanos, the conservative People's Party and Vox To reach a majority scenario lies within the error rate of the surveys and can not be ruled out.
With the trauma of the military dictatorship under Francisco Franco, who died in 1975 and was still remembered by his older generation, Spain was for a long time viewed as a resistance to the spreading wave of nationalist, populist parties much of Europe.
But this time, Vox will get seats due to voter dissatisfaction with traditional parties, a focus on widespread anger at Catalonia's pro-independence aspirations and non-mainstream views that include opposition to a law against gender-based violence, is reinforced.
One of several unknowns is how big Vox's entry into Parliament's lower house will be. Opinion polls have made a wide range of predictions and have struggled to determine the electoral base of the party.
The high number of undecided voters-even four in ten in some polls-has made predicting the outcome as difficult as complicating a complex electoral system in which 52 electorate elect 350 legislators.
This system has not been tested in Spain's new political era, marked by the final end of the long-dominant PP and Socialist dominance.
Voters in the depopulating rural core areas – many of them are old and may feel little connected directly to the young, male, urban political elite in the country – are of particular importance.
You are proportionally choosing more legislators than inhabitants of big cities. At the same time, the border point for parliamentary representation is more difficult to achieve, which makes the outcome more difficult to predict the more parties there are.
An opinion poll will be published at 20.00. The results will flow through the evening and almost all votes will be counted until midnight. In the last two elections, the 20 o'clock polls did not give an accurate picture of the final result.
Additional coverage by Belen Carreno; Letter from Ingrid Melander; Edited by John Stonestreet