A poll carried out in November by the central government's influential Sociological Research Centre (CIS) predicted that the three pro-independence parties would get only up to 67 parliamentary seats out of 135, just under the absolute majority of 68.
The lawyers for Catalan ex-president Carles Puigdemont and four of his separatist allies say that the five will be judged on whether they can be extradited from Belgium to Spain on December 14, exactly one week before a key regional election in Catalonia in which they are all running for re-election.
Mr Puigdemont's defence lawyer, Paul Bekaert, said on Monday the prosecutor sought the extradition of the five but Mr Bekaert insisted that the Spanish charges are not punishable in Belgium and thus were no grounds for extradition.
They are fighting return to Spain to face rebellion, sedition and embezzlement charges that can be punished with decades in prison under the Spanish law.
"We also highlighted the danger for the impediment of their human rights in Spain", he said.
With political campaigning not set to get underway until midnight, Catalan pro-independence groups held protests in front of town halls to oppose the Supreme Court's decision to keep some of the independence movement's leaders in custody.
Political parties in Catalonia started campaigning Monday for the early regional election called by the Spanish government in hopes of quashing the region's independence movement and featuring candidates who are either in jail or living overseas to avoid detention.
The Supreme Court judge also upheld custody orders without bail for the former regional interior minister, Joaquim Forn, and the leaders of Assemblea Nacional Catalan and Omnium Cultural, two grassroots groups that have been the main drivers of the separatist bid in Catalonia.
Madrid called the new elections after the independence declaration, while dismissing Catalonia's government and suspending the region's autonomy.
"No matter what, they will be [in Brussels] till at least December 21 and according to my calculations this could go on till mid-January", lawyer Jaume Alonso Cuevillas told Catalan radio Rac1, referring to Puigdemont and the four former ministers.
Prosecutors last month asked the judge to approve the European arrest warrant issued by Madrid for the five in the opening round of what could become a protracted courtroom battle.
"I am convinced that no matter what happens they will have recourse to an appeal", the lawyer said.
Any release on bail would mark a turn in the election campaign, particularly for separatist parties who have repeatedly accused Madrid of taking "political prisoners" and "repression" after their attempt to declare unilateral independence fell flat.
Whatever decision is made on Monday, two appeals will be possible and a final ruling could well only come only after the vote.