Catalan voters will decide Thursday whether to return the separatists to power or to bring in a pro-unity government, as their region's independence crisis nears its moment of truth.

Here are the possible outcomes of the December 21 regional vote:

- A pro-unity revolt? -

Ciudadanos, a centrist anti-independence party formed in 2006, could win, with its charismatic 36-year-old candidate Ines Arrimadas taking the reins of power in the wealthy region.

Such an outcome would be momentous for Catalonia, where nationalists have dominated since democracy was reinstated in Spain following the death of dictator Francisco Franco in 1975.

Arrimadas has pledged to enter into "dialogue" with her rivals, in a bid to overcome the divisions in Catalan society.

She has also promised to focus on social problems, which she claims were ignored under axed president Carles Puigdemont.

A regional government led by Ciudadanos would likely please the ruling conservative Popular Party (PP) in Madrid, which has stopped at nothing to put the brakes on the independence drive.

But ultimately, it could also turn into a national threat to the PP, attracting voters in search of an alternative to the bipartisan politics that have long dominated Spain.

- A president in exile? -

Puigdemont left for Belgium at the end of October. He now faces charges of sedition, rebellion and misuse of public funds over his government's independence drive and could be jailed before his trial.

But in a surreal twist to the crisis, he announced his candidacy from exile, pledging to recover the "dignity" of the Catalan people after being "humiliated" by Madrid.

Victory for Puigdemont would be a blow to Spanish Prime Minister Mariano Rajoy, who deposed him after the Catalan parliament declared independence on October 27 following a banned referendum.

While Spain eventually dropped an international arrest warrant for Puigdemont, he will still be detained if he returns to the country, raising the question of how he might govern should he win.

- A leader behind bars? -

Oriol Junqueras, deputy president under Puigdemont's deposed leadership, was remanded in custody on November 2 over his role in the independence bid.

His ERC party has a narrow lead over Ciudadanos in the polls, and should it win, secretary-general Marta Rovira would likely be tasked with running the day-to-day affairs of government.

Rovira, however, also faces the threat of detention.

Earlier this month, the Supreme Court in Madrid ordered an expansion of a probe to target members -- including Rovira -- of a "strategic committee" allegedly set up to achieve independence in Catalonia.

- Deadlock? -

With none of the leading candidates expected to win a majority, any one of the winners will need to secure backing from rival parties in order to rule.

But in an election seen as make-or-break by all the contenders, there is a risk that no one will come forward to support a winner, opening the way to lengthy negotiations, followed by deadlock.

"Even within the independence camp, the formation of a future government will be riddled with complexity," said Pablo Simon, political science professor at Madrid's Carlos III University.

Any secessionist winner would need the backing of the radical leftist CUP party, which demands immediate, unilateral independence for Catalonia.

Ciudadanos, meanwhile, would need the Socialists and the PP's support -- and even then it might not have a parliamentary majority.

- A Socialist compromise? -

"The possibilities of deadlock and fresh elections are very high," according to political analyst Pepe Fernandez-Albertos of the CSIC think-tank in Madrid.

One way forward, he said, could be for pro-unity parties to rally behind Miquel Iceta, the Socialist candidate.

He is seen as having more potential for dialogue than the others, and he has pledged to seek an amnesty for the secessionist leaders.

Such an outcome would mirror a solution put in place in the Basque Country, which was crisis-hit for decades, where the Socialist Patxi Lopez led a minority government from 2009 to 2012.