The biggest difference between this weekend's crucial Spanish elections and ballots past: It's no longer a two-horse race.

In less than two years, the business-friendly centrist group, Ciudadanos, and the hard left anti-austerity Podemos party have sprung up, gathered force and now pose a serious challenge to the ruling center-right Popular Party and the leading opposition Socialists — who have alternated in office for nearly four decades.

The newcomers have nothing in common except striking a chord among millions of Spaniards, especially those under 35, who are angry and disillusioned over soaring unemployment, harsh austerity measures and the impunity that politicians and business representatives seem to enjoy amid incessant corruption cases.

Here's a looks at the upstart parties and their mainstream foes:

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PODEMOS

Podemos has provided the biggest wake-up call in the new political landscape. Led by a pony-tailed political science professor, the party was founded only last year and has taken Spain's youth vote by storm. It has roots in the March 15, 2011 sit-down protest movement launched in Madrid by people angry over political and financial corruption.

It won five seats in the European Parliament in 2014 in its first election, just four months after being formed.

But while it was briefly seen as rivaling the ruling Popular Party and the opposition Socialists in popularity, much of Podemos' glitter has worn off, and it's now regularly placed fourth in opinion polls.

The party talks of rolling back health and education cuts, reducing the working week and re-nationalizing formerly state-owned large companies. It has also suggested restructuring debt held by ordinary people and small businesses, as well as Spain's public debt.

Podemos has supported some of the policies of left-wing governments in Venezuela, Bolivia and Ecuador, making many mainstream politicians at home bristle. In Europe, it openly backed the anti-austerity challenge to creditors by Greece's far-left governing party, Syriza.

The party also openly opposes the bombing campaign against the Islamic State group.

It is the only national group supporting a self-determination referendum in the economically powerful northeastern region of Catalonia, where independence parties threaten to break away from Spain in 2017.

The party is seen as chiefly robbing votes from the Socialists and the small United Left coalition.

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CIUDADANOS

Since presenting itself on the national stage a year ago, Ciudadanos has rocketed in popularity and is now the party with the best chance of breaking the mainstream parties' grip on power.

Headed by the dapper Albert Rivera, Ciudadanos grew out of a gathering of intellectuals in Catalonia opposed to the region's drift toward independence — and who felt Spain, in Rivera's words, was "rotten with corruption."

Since then the party has spread across the country, making inroads in local and regional elections. Like Podemos, Ciudadanos owes much of its success to the oratory prowess of its leader. His clean-as-a-whistle image, promise of transparency and zero tolerance for graft has clicked with many, especially those born under democracy, restored in 1978 after the death of dictator Gen. Francisco Franco. These voters are also tired of the established leftist-or-rightist party split that has dominated Spanish politics since the 1936-1939 civil war.

The party promises to maintain the country's strong social welfare system while cutting taxes, reducing bureaucracy, investing in research and development and luring back well-educated Spaniards who left the country during the nation's 2008-2013 downturn.

It advocates merging small town councils and eliminating the Senate, a separate a parliamentary chamber chiefly representing Spain's provinces, but which is seen having little or no necessary legislative function.

Ciudadanos is the only Spanish party that favors sending troops to help fight the Islamic State group.

It is seen as chiefly taking votes from the ruling conservatives.

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THE POPULAR PARTY

Headed by Prime Minister Mariano Rajoy, the center-right party won an absolute majority of 186 seats in 2011 but its handling of the severe economic crisis — including tough austerity measures — and myriad corruption cases have whittled away its support. Most polls tip it to win again but it is likely to need a pact if it wants to stay in office. No party has shown any willingness to lend it support.

Although the country is still suffering 21.2 percent unemployment, the party claims the economic recovery of the past two years is due to its policies and reforms, and hopes this will help it gain re-election.

The party's legalistic stance in dealing with the Catalan separatist drive, and its introduction of legislation seen by some as violating civic rights, have also alienated many supporters.

Founded in 1989 by revamping a previous party called the Popular Alliance, the party has always been linked to the Catholic Church and is regarded as having roots in the Franco dictatorship. It has won three of the 11 elections held since Spain returned to democracy.

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THE SOCIALISTS

Headed by Pedro Sanchez, the Spanish Socialist Workers Party, or PSOE as it is known in Spain, was founded in 1879, making it Spain's oldest party and one of Europe's first socialist parties.

The party went into exile during the dictatorship. It has won six of the 11 elections held since Spain returned to democracy and is generally credited with transforming the country from a backward authoritarian state into a dynamic European Union democracy.

Under former prime ministers Felipe Gonzalez and Jose Luis Zapatero, it gained EU membership, stayed in NATO and introduced many modernizing infrastructure projects and social reforms, such as widening the abortion law and legalizing gay marriage.

The party's popularity plunged during its last spell in office, between 2004 and 2011, chiefly because of Zapatero's blindness to the looming economic crisis that pushed unemployment sky-high and the economy close to a meltdown.