A series of polls on Sunday and Monday put 'Unidos Podemos' in second place overall.
The anti-austerity firebrand will forgo a deputy PM role if it blocks a progressive government.
The country's ongoing political deadlock is taking its toll.
'Winter is coming' and there is no new Spanish government in sight.
Spaniards will cast their ballots on December 20 after a turbulent few years for Spanish politics.
Pablo Iglesias is on a mission to save Spain.
Pablo Iglesias made the challenge at his 'alternative state of the nation' address on Wednesday.
It will be the party's first major event in the Spanish capital.
Spain's far-left party have declared their intent to follow in Syriza's footsteps after election triumph.
Several members of the ETA have spoken of their support for the grassroots political party
20-year-old 'Little Nicolas' allegedly impersonated government agents, politician's aides and shook hands with the King
Despite the radical left-wing party taking the lead in the Spanish polls, over 50% don't see them as a realistic alternative to the two main parties
Hackers have targeted pro-independence campaign sites and local Catalan government web pages, while crank calls have kept organiser's line busy
Six politicians, including members of the Zapatero administration, allegedly used government funds to acquire works by the Spanish artist at inflated prices and whilst taking a fee
The 10-month-old left-wing party leads the polls, in the biggest ruction of the two-party system since the fall of Franco
As Europe's anti-elite rebellion gains pace, an energetic new Spanish party is surging into the limelight - and becoming a genuine contender for next year's election