Spain’s political class can find no respite from the attention of prosecutors, and no shelter from the storm of popular indignation. Over the past few days, mayors, regional bosses and party functionaries have found themselves once again in the back of police cars, heading to court for questioning – and in some cases straight to prison.

The latest case concerns allegations that local politicians in Madrid, León, Murcia and Valencia took kickbacks from the private sector in return for awarding public contracts worth an estimated €250m. Thirty-five officials were arrested on Monday, including the former number two of the ruling Popular party in the Madrid region.

What has shocked Spaniards are not so much the details of the scandal but the fact that it has surfaced so shortly after other affairs highlighting the apparent rapacity of parts of the country’s political elite. This month, newspapers were filled with details of the “phantom” credit cards awarded by a now-defunct regional savings bank to political leaders in Madrid. Some politicians spent hundreds of thousands of euros on luxury holidays, three-star restaurants and fancy clothes.

Kickbacks, illegal donations, tax fraud, embezzlement – the alleged cases stretch from Catalonia in the north to Andalucia in the south, and cover almost the entire spectrum of Spanish politics. In a rare gesture of public contrition, Mariano Rajoy, the prime minister, offered an apology on Tuesday to “all Spaniards” for his party’s involvement in the latest affair.

Unsurprisingly, the string of cases has prompted much soul-searching and a frenzied debate over the origins of Spain’s corruption problem. Analysts agree the country’s decade-long property boom played an important role, ensuring there were plenty of lucrative public contracts to award, and desirable slices of land to rezone and sell to property investors.

The glacial pace of Spain’s justice system is another factor, as is the high degree of politicisation in the public prosecution service. It usually takes many years for corruption probes to end in a sentence, if they do at all. Despite the recent flurry of judicial activism, it is hard to shake off an impression of impunity.

Perhaps the most striking cause of Spain’s corruption problem, however, is the fact that voters seem so reluctant to punish corrupt politicians where it really matters: at the polls. Take the examples of Andalucia, a bastion of Spain’s Socialist party, and the Valencia region, where the ruling PP has long held sway. Both have suffered massive corruption scandals in recent years, leading to numerous arrests and resignations. Yet latest polls show that, despite heavy losses, the ruling party in both regions remains in the lead.

Elena Costas-Pérez, an economist at Autónoma university in Barcelona, has studied the phenomenon of Spain’s forgiving voters by analysing the results of local elections over the past decade. Her doctoral thesis concludes that citizens are less likely to vote for mayors affected by corruption scandals – but not by much, and only if there is strong media coverage. “All too often these effects are not big enough to be a threat to the re-election of corrupt mayors,” she writes.

Ms Costas-Pérez suggests Spain’s polarised media are at least partly responsible for this situation because they tend to focus on those corruption scandals that affect “the other side”. It is an argument that is easy to expand into a broader observation about the polarisation of Spanish politics: the gulf between the right and the left is so deep that many voters seem ready to hold their nose and stick with their party despite corruption scandals. Crossing the left-right divide is, for many voters, simply not an option.

The recent rise of Podemos, a leftwing protest party that is riding high in the polls, may change that dynamic. It campaigns aggressively against Spain’s “political caste” and advocates wholesale reform of the country’s political and economic system. It is a message that is proving hugely popular, and that may yet succeed in turning Spain’s dual-party regime upside down.

Looking at the recent past, however, it is hard to escape a simple conclusion: Spanish voters are furious with their political leaders, and care deeply about the apparent wave of corruption cases – but often not quite enough to vote for the other guy.

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