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Spain could be set for a political upheaval in key local elections this weekend, with strong signs that voters fed up with economic crisis and corruption scandals may punish both the ruling conservative Popular Party and the leading opposition Socialists.

Opinion polls agree on one point: With centrist and hard-left upstarts rocking the boat, the elections are likely to herald the end of Spain's dominant two-party system that has seen the heavyweights alternating in power for nearly four decades. The weekend battle is seen as a bellwether of general elections later this year.

The business-friendly Citizens party and the radical leftist We Can group began operating as national parties only last year. V0ters see them as a welcome breath of fresh air, and many polls indicate they may finish neck-and-neck with their established rivals.

"Spain faces a serious problem of government stability because there is no tradition of pacts or coalitions among parties," said Maria Jose Canel, a political communication professor at Madrid's Complutense University.

Campaigning ends at midnight Friday. Polls open Sunday morning and close in the evening, with final results expected by midnight. At stake are seats in more than 8,100 town halls and parliament seats in 13 of Spain's 17 regions.

The biggest and most symbolic battles will be for control of Madrid and Barcelona.

In the capital, the Popular Party looks set to lose the majority it has held since 1991 while in the Catalan port city, a popular anti-eviction campaigner backed by We Can could oust the region's long dominant and conservative Convergence and Union party.

A poll by the government-run Center for Sociological Research earlier this month indicates that Prime Minister Mariano Rajoy's Popular Party may remain the party with the most votes, but will lose its majority in 11 of the 13 regions.

Spain, the European Union's fifth-largest economy, was battered by the continent's crisis for six years, and emerged from a double-dip recession only at the end of 2013. Although the economy has grown for the past seven quarters, is among the fastest-growing in the EU, and among those creating most jobs, the country still has a staggering 24 percent unemployment rate. It is more than double that for people under age 25.

Reforms that made it easier and cheaper to lay off workers, educational and health cutbacks and tax hikes have angered many Spaniards. Equally upsetting for them is the seeming impunity of mainstream party politicians and their business cohorts embroiled in scandals.

Rajoy hopes the recent turnaround in economy can stem the party's plunge in popularity. Besides its grip on municipal and regional power, the party holds the strongest majority a party has ever held in the parliament.

"When you have flown that high, the fall can be great," said Canel. "The Popular Party will be punished. The punishment may be less than expected but it's sufficient if they lose their absolute control in the majority of regions."

While nothing indicates Spain will follow Greece — where a new coalition of the radical left and nationalist right took office in January — Citizens and We Can are nonetheless tipped to become powerbrokers in many towns and regions.

"The crisis has modified the way Spaniards view politics," said Canel. "Before, voting was ideological, between the right and left. Since the crisis people tend to go more by policies and what parties offer, rather than stick with those they have always voted for."

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Associated Press writer Ciaran Giles in Madrid contributed t this report.