Lawmakers on Tuesday will debate Alberto Nunez-Feijoo's bid to become Spain's next prime minister a day before an inauguration vote.
With the required support to obtain 176 votes, Feijoo is likely to use the chance to attack Pedro Sanchez's efforts to stay on as prime minister by courting a hardline Catalan separatist party cast in the role of kingmaker.
A month after Spain's uncontested July election, King Felipe VI tasked Feijoo with forming a new government ahead of an investiture vote on September 27.
Although he has gained the support of the far-right Vox plus a handful of other seats, Feijoo has found himself four seats short, with regional parties rejecting any alliance that would include Vox over its hardline opposition to Spain's system of devolved regional politics.
The demands of regional parties, t intend to give in to blackmail, Feijoo told El Mundo daily on Monday, acknowledging he faced almost certain defeat in Wednesday's vote.
The debate may allow Feijoo to outline an alternative programme that would be a big contrast from what Sanchez is proposing for the future, said Astrid Barrio, a political scientist at Valencia University.
The 62-year-old has spent the last few weeks attacking Sanchez, for the likely concessions he will need to make to the hardline Catalan separatist JxCat party to stay in power.
That was the central theme of the Sunday mass protest he called in Madrid under the mantra :
Carrying Spanish flags and banners, around 40,000 people, according to the organisers and the Madrid authorities, gathered two days before the debate on Feijoo, who currently does not have a majority, becoming head of government.
JxCat's primary demand for its seven key votes in support of Sanchez is for an amnesty for hundreds of activists facing legal action over the 2017 failed Catalan separatist bid, which sparked Spain's worst political crisis in decades.
An amnesty has no place within the Constitution, Feijoo said on X, formerly Twitter, that it is an unjustified and unethical attack on the rule of law and the separation of powers because it defies rulings handed down by the courts.
The parliamentary debate begins Tuesday with a speech by Feijoo, followed by a first vote on Wednesday when he will need to secure 176 votes in favor.
If he fails, he will face a second vote on Friday, when he needs a simple majority of more votes in favor than against.
Barring any unexpected surprises, Feijoo is not expected to pass either vote, giving Sanchez a chance to piece together a government.
Spain will face new elections in January if Sanchez is unable to pass an investiture vote.
To pass the elections, Sanchez is banking on support from two Catalan separatist parties which both supported his candidate for parliamentary speaker in a vote last month.
They had requested that lawmakers be allowed to speak in Catalan, Basque and Galician when addressing Spain's parliament, which came into force last Tuesday.
But approving an amnesty would be an extremely sensitive political issue that would affect Catalan separatist leaders who fled Spain to avoid prosecution over the independence bid like Carles Puigdemont, the leader of JxCat.
Approving amnesty is not only a red line for the right, but also for elements within Sanchez's own Socialist party.
Although Sanchez, who had been jailed over the failed bid to seize Catalan territory in 2021, has not spoken publicly about the amnesty issue.