“I AM often asked by people from India and Russia, ‘Why do you worry?’”, says David Lewis, head of Corruption Watch, a privately funded watchdog based in Johannesburg. Why indeed? South Africa sits in the middle of international rankings of corruption. It is not Sweden but nor is it Zimbabwe. It has a free and impertinent press. Its constitution gave birth to bodies that check and balance the powers of government. And its judges back their freedoms to poke the executive in the eye.

Indeed, the ruling African National Congress (ANC) has been checked-and-balanced recently rather more than usual. The public protector, a body created by the constitution, published a report last month that chastised the president, Jacob Zuma, as partly responsible for the 246m rand ($24m) lavished on his private home in Nkandla, in KwaZulu-Natal. When the Democratic Alliance, the main opposition party, sent a text message to 1.5m voters saying the president stole your money, a court upheld its right to do so; the ANC is appealing against the judgment. Another court said Zwelinzima Vavi, a vocal critic of ANC corruption, should be reinstated as boss of the biggest union federation.

South Africa has plenty of bodies to be proud of beside its courts. Its treasury is a bastion of rectitude. The revenue service is the envy of richer countries for its ability to harvest tax. The central bank recently raised its main interest rate, despite a weak economy (and a general election next month), to show it is serious about fulfilling its main remit of controlling inflation. The country’s big banks are well regulated and conservative. Its biggest firms are respected multinationals. As long as such institutions stay strong, reckons Mr Lewis, South Africa will not be prey to the sort of endemic corruption seen in Russia, China or India.

Yet there is plenty still to fret about. Start with the Nkandla scandal. Mr Zuma did not quite thumb his nose at the public protector but he came mightily close. Her report recommended that Mr Zuma apologise, pay for some of the refurbishments to his home and report to parliament within two weeks. When the deadline day arrived he airily sent a three-page letter to Parliament’s Speaker, declaring that the report’s findings were at odds with a cabinet investigation, which cleared him of wrongdoing. And he said he would await a third report before giving a further response.

The stalling is not surprising with elections due on May 7th. But the scandal is corrosive not just because of the sums involved but because of the damage it does to South Africa’s institutions. It sullies the presidency as well as the president. And it does broader harm. People feel freer to be corrupt if they sense that people at the top are getting away with it, says Mr Lewis. His biggest worry is the weakness of the police and prosecutorial services. If corruption goes unpunished, it will spread. Most South Africans reckon that corruption at all levels is growing apace, especially in contracts for public works, where lines to the ANC seem increasingly necessary.

And it is hard to avoid an impression that the politically connected are somehow immune to prosecution. The main prosecuting authority is appealing against a court judgment that compels it to indict a former head of the police’s criminal intelligence for fraud, corruption and murder. Meanwhile an investigation led by a senior judge into misdeeds surrounding a big arms deal in 1999 seems to be going nowhere. Mr Zuma’s financial adviser was convicted in 2005 for soliciting a bribe on his behalf from a French arms company. Related charges against Mr Zuma were dropped on the eve of his election as president in 2009. The reasons for that are still far from clear.