“There are a lot of possible outcomes that could lead to a very long negotiation, lasting perhaps weeks if not months,” he added.
The vote will be Catalonia’s second since October to be held in extraordinary circumstances that have been used by both sides in the dispute to raise fundamental questions of democratic legitimacy.
The first vote, the independence referendum on Oct. 1, was declared illegal by Spain’s courts and central government in Madrid, which sent thousands of police officers from outside the region to block it.
Even though there were clashes at polling stations and opponents of secession largely refrained from voting, the result was declared an overwhelming victory by Catalonia’s separatist leaders and used by the regional parliament as the basis for its independence declaration.
Mr. Rajoy’s answer — direct administration from Madrid under emergency measures and new elections to shake up the regional parliament — has in turn been criticized by his separatist opponents as an autocratic abuse of authority.
Among the biggest questions is how separatist politicians who are being prosecuted by Spanish authorities for rebellion could take their seats if they win. Some remain in prison. Others are free on bail but face 30-year sentences.
Carles Puidgemont, whom Mr. Rajoy dismissed as leader of the region, has been campaigning from Belgium, from where he has refused to be put on trial in Spain.
Despite early concerns that separatist politicians, parties and voters would boycott a vote considered illegitimate by some, the current consensus is that turnout may actually be at a record high.
The fear of being politically sidelined has prompted all sides to enter the fray, as politicians have cast the election as a make-or-break one for Catalonia.
The exceptional circumstances of the election have polarized both politicians and the public. The most recent polls show the main unionist and separatist parties neck-and-neck, with each side possibly falling narrowly short of a parliamentary majority.
The follow-up negotiations to form a government could prove complicated. The main separatist parties, whose majority was already fragile, are no longer running on the joint ticket that brought them into office in 2015.
Should they win, they have not agreed on how to revive a coalition that has been further strained by the turmoil of the last few months and their botched attempt to declare independence.
Such a fractured outcome could push to the fore more centrist politicians who might help bridge the gap between the feuding separatist and unionist parties at the extremes.
One possible power broker is Miquel Iceta, the Socialist leader in Catalonia, who is known to enjoy dancing during his party rallies but has shed his ebullient approach to convince voters that he can be Catalonia’s kingmaker.
Mr. Iceta’s chances of winning are remote. His Socialists are expected to take only about 15 percent of the votes, according to the latest opinion polls.
But such a result could still allow Mr. Iceta to take charge of Catalonia as a compromise choice, if the vote has no clear outcome.
“The independence movement has polarized everything, and the center ground has disappeared, either because its representatives have disappeared or have become radicalized,” Mr. Iceta said in a recent interview.
“We’re making an effort to reconstruct this center space for the moderates,” he said.
That is no easy task. Mr. Iceta himself opposes independence and is against a referendum, even if the terms were set by Madrid.
He compared it to a penalty shootout in soccer — with the significant difference being that, in soccer, any team can hope for a different result in the next competition.
With independence, “the problem is that you create a definitive divide in a society over an issue that is as sensitive and irreversible as this,” he said.
He argues that the answer to Catalonia’s crisis is more autonomy within a new Spanish federal structure. But, nationally, his Socialist party endorsed Mr. Rajoy’s emergency takeover of Catalonia on Oct. 27.
Mr. Iceta’s suggestion that Mr. Rajoy now make important concessions has antagonized both the central government and many within his own party, giving him a difficult needle to thread.
Recently, when Mr. Iceta suggested that Catalonia should be forgiven part of its debt as part of a broader political settlement, other Socialist politicians — some from poorer regions — balked.
Similarly, Mr. Iceta was criticized for proposing that separatist politicians should be pardoned rather than put on trial, if they acknowledged that a unilateral declaration of independence was illegal. Last week, he withdrew the proposal as “premature.”
Mr. Iceta has sought to distance himself from other unionist parties, including Ciudadanos, which has been leading in most opinion polls, antagonizing them as well.
Last weekend, he attacked Albert Rivera, the leader of Ciudadanos, as a “shoddy democrat” for his uncompromising stance.
His relations have been no better at the other extreme with Podemos, the only major national party that advocates allowing Catalans to hold an independence referendum.
The party’s stance has infuriated Mr. Rajoy and other mainstream politicians in Madrid, who maintain that such a referendum would be illegal.
Even though polls suggest that the Catalan branch of Podemos will struggle to reach 10 percent support on Thursday, the party has made some of its biggest advances in Catalonia, and it too could play a pivotal role in post-election haggling.
Politicians from Podemos say that Mr. Iceta is making empty promises while opening the door for Mr. Rajoy to extend his takeover of Catalonia.
“It’s hard to understand why somebody like Miquel Iceta on the one hand defends dialogue and on the other hand gives in to the parties that just want to re-centralize Spain,” said Gerardo Pisarello, an official from Barcelona’s city hall, which the Catalan branch of Podemos controls.
Still, not everyone is writing off the possibility that Mr. Iceta, or another politician, could find common ground in Catalonia, even if Thursday’s election proves divisive.
Mr. Iceta “is perhaps the only one who can actually discuss and engage with both sides, without being labeled completely either white or black,” said Jordi Alberich, the director general of the Cercle d’Economia, a Catalan business association.
“Since it seems so difficult for one side to win over the other on Thursday,” he added, “Catalonia is in desperate need of more moderate politicians.”