Nigeria’s president, who came to power with a pledge to tackle corruption, has said he needs more time to sort out the problem but has begun no successful prosecutions and has appeared to condone colleagues tainted by serious allegations.

Muhammadu Buhari was elected four years ago on a wave of disgust at the perceived avarice of Goodluck Jonathan’s administration – but as he fights for a second term in Saturday’s election, his efforts to bring corruption under control are widely seen as a failure.

Thousands of government supporters carrying twig brooms filled the streets of Surulere in Lagos, Nigeria’s southern megacity, on Saturday, waving them in the air to show their backing for the president and his anti-corruption drive.

“It’s a broom – broom revolution,” thundered Bola Tinubu, a ruling party grandee, addressing the crowd that had gathered for Buhari’s “mega-rally”. “Broom broom!” he continued, imitating a motorbike as his fans swept the sky.

Mai Gaskiya – Mr Honesty – has long been the ascetic-looking Buhari’s nickname, and his personal reputation as a lone squeaky-clean man in a sea of corrupt Nigerian politicians has survived his first term.

“This is the only president no British prime minister can call a thief,” said Adams Oshiomhole, chairman of Buhari’s APC party, referring to the time David Cameron was caught on camera on the eve of anti-graft summit telling the Queen that Nigeria was “fantastically corrupt”.

At the summit, Buhari was asked if his country really was as corrupt as Cameron portrayed it. “Yes,” he said simply, after a pause.

But nearly three years later, rumours of corruption are again swirling around the president’s men.

Supporters Buhari hold up brooms at the Lagos rally. Photograph: Sunday Alamba/AP

None of the many corruption cases initiated by the administration has ended in a conviction. A former secretary to the federation who allegedly diverted 270m naira (£578,000) of funds meant for people displaced by Boko Haram to accounts linked to him was only charged last month, two years after being indicted by the Senate. It took months of pressure from the press and parliament for him to be fired; he is currently one of the leaders of the president’s campaign in his home state of Adamawa.

Quick Guide 2019 Nigerian elections Show On Sunday 16 February, 84m voters will choose who leads Africa’s largest democracy. Despite over 60 candidates, it is likely to be a choice between President Muhammadu Buhari, and former vice president Atiku Abubakar. President Muhammadu Buhari (APC candidate) Buhari's 2015 victory with his All Progressives Congress (APC) party was built on three promises: to rid Nigeria of endemic corruption, fix the economy and tackle security threats. The government says it is making progress in the fight against Boko Haram, but it is now in its tenth year, and the economy entered and climbed out of recession under Buhari. Opponents say his government is failing to tackle corruption, while Amnesty International say the army has been responsible for human rights abuses. After spending five months in Britain in 2017 receiving treatment for an undisclosed ailment, opposition groups said he was unfit for office, but Buhari, 76, says he is strong enough to serve. Atiku Abubakar (PDP candidate) Main opposition People's Democratic Party (PDP) candidate Atiku, 72, has been caught up in corruption allegations since serving as vice president from 1999-2007. He has repeatedly denied any wrongdoing. He has promised business-friendly policies to double the economy to $900bn by 2025. He wants to privatise parts of the state oil company, and create a $25bn fund to support private sector investment. Atiku's opponents have claimed he would exploit those pro-business policies to enrich himself and those around him. A divided country Nigeria is deeply divided between the mainly Muslim north and largely Christian south. There are more than 200 distinct ethnic groups. This has led to an unofficial power-sharing agreement, with the presidency alternating between north and south every eight years. It isn't just religion or ethnicity causing fractures. Half of Nigeria’s registered voters are aged 18-35. Many say their ageing leaders are out of touch, and supported a "Not Too Young to Run" campaign to encourage younger people seeking office. Election concerns 2015 was the only time Nigeria had a peaceful power handover since civilian government took over in 1999. Even then there was evidence of vote buying, voter intimidation, and other forms of corruption. International observers fear election interference and rigging this time around. Last month, Buhari triggered a constitutional row when he suspended the chief judge, who has a crucial say in resolving election result disputes. Path to victory The candidate with the most votes is declared winner, as long as they have at least one-quarter of the vote in two-thirds of Nigeria's 36 states and the capital. Otherwise there is a run-off. Martin Belam and agencies Photograph: John MacDougall/AFP

At least two ministers had petitions against them for misappropriation of funds as governors of Lagos and Rivers, the two biggest state economies. Last October, Oshiomhole, also a former state governor, was served papers by a federal high court for fraud perpetuated in office; a month later, the secret police interrogated him for collecting bribes to subvert party primaries nationwide.

Transparency International said Nigeria had not moved in its latest corruption perception index released last month.

Supporters of All Progressives Congress attend a campaign rally. Photograph: Reuters

Buhari did implement the Treasury Single Account initiative, making it more difficult for officials to steal money – something his predecessors had failed to do. But meanwhile, it is unclear how the $1bn (£775m) he released from the country’s Crude Excess Account to fund the fight against Boko Haram has been spent.

There has also been no investigation of Abdullahi Umar Ganduje, the Kano governor who was secretly filmed stuffing wads of dollars into his robes – allegedly a large bribe in exchange for lucrative government contracts. On the contrary, Buhari has been happy to appear in public with the governor, and came in person to endorse his campaign for re-election in Kano, a traditional APC stronghold – which, with a population bigger than Belgium, is the biggest state in Nigeria.

“He is a responsible man,” the president said of Ganduje, perhaps thinking wistfully of 2015. Then, Ganduje’s powerful predecessor, Rabiu Kwankwaso, delivered 1.9 of the 2.1 million votes in Kano for Buhari - something Ganduje is now expected to do, as Kwankwaso has defected to the opposition.

The Peoples Democratic party presidential candidate, Atiku Abubakar, (centre) waves to supporters at a campaign rally in Calabar, Nigeria. Photograph: Tife Owolabi/EPA

Buhari’s own enduring reputation as a “good man” of unimpeachable character is not matched by his main challenger, former vice-president Atiku Abubakar. Atiku, as he is popularly known, has repeatedly denied multiple corruption allegations against him, including that he diverted $125m of public money to his own business interests and that his American wife helped him move $40m of “suspect funds” to the US, which was the subject of a Senate report in 2010. Neither allegation has come to court, but Atiku has not done much to convince Nigerians that he has changed.

“I am not going to enrich members of my family but my friends. Are my friends not entitled to be enriched?” he asked a meeting of Lagos businessmen last month, only half joking.

Rather, Atiku is seen as a good businessman, capable of running the economy and making good on his promises to create jobs and shrink a bloated government, outweighing the negative impact of any corruption.

With vast fertile lands, enormous natural resources and a strong youth population, Nigeria has enormous potential, but successive governments have failed to diversify the economy, so it is almost entirely dependent on the sale of its crude oil. It is this money that is siphoned off by corrupt officials – for example, in the sale of an oil block possibly worth $6bn to Shell and Eni, for which hundreds of millions of dollars in bribes were allegedly paid. The companies deny any illegality and the court case is ongoing.

There is nothing inherently corrupt about Nigeria or Nigerians. Most express their exhaustion at the constant theft of their resources.

The country was an invention of the British empire, which decided to create a state out of hundreds of ethnic groups to make administration easier. Partly as a result, national unity is in short supply, and the country’s bureaucracy has had a tendency towards self-enrichment.

Today, Britain is a key destination for corrupt officials’ ill-gotten gains – in 2016 Buhari’s anti-corruption chief said $37bn of stolen money had been routed through London in two years.

But the thousands of brooms – including a giant statue of one that was put up at Abuja’s city gates on Saturday – have yet to put a dent in the dust of Nigeria’s corruption, despite the best efforts of the ruling party’s propagandists.

“President Buhari has made every part of this country safe again and he has stopped corruption dead in its tracks,” proclaimed a radio advert on Lagos’s airwaves ahead of the rally.

“Share facts, not rumours,” said an advert sent via WhatsApp immediately after.