Despite eight years of civilian rule, the Nigerian government can't seem to shake off its autocratic habits. Under military leaders, presidential decrees were the norm. But today President Olusegun Obasanjo's administration appears to be selectively interpreting the law to similar effect, this time to eliminate scores of opposition candidates from the national and state elections in April.

Obasanjo, who is expected to step down in May after the elections, has presided over Nigeria's longest-ever stretch of uninterrupted civilian rule. Last year he tried to amend the constitution to allow himself to stand for a third term, but was defeated in the face of unusually strong opposition led in part by his own vice-president, Atiku Abubakar. This battle brought long-simmering tensions between the president and his deputy to a head. Since then, each of them has busied himself leaking purported evidence of the other's corruption to the press. Atiku deserted the ruling People's Democratic Party and now seeks to succeed Obasanjo in office as a member of the opposition.

Atiku has been informed by Nigeria's Independent National Electoral Commission that he would not be on the ballot because of an administrative "indictment" on charges of corruption. The "indictment" was issued by a special panel set up in September which confirmed charges brought against Atiku by Nigeria's corruption watchdog, the Economic and Financial Crimes Commission (EFCC).

The vice-president is not alone in this predicament; 37 other candidates have also been disqualified from running after being "indicted" by a panel that deliberated for only 48 hours after the EFCC presented them with a list of allegedly corrupt politicians. Conspicuously absent from the list are several senior Nigerian politicians previously named by the EFCC, including the vice-presidential candidate for Obasanjo's party, Goodluck Jonathan.

Corruption in Nigeria remains an enormous obstacle to fighting poverty and making government accountable. It's hard to argue that barring corrupt politicians from holding office would be a bad thing. But Human Rights Watch is worried that the presidency appears to be using corruption allegations selectively to frustrate the ambitions of his political opponents.

Nearly all the people who have been barred from the elections are either members of the opposition or supporters of Atiku within the ruling party. The "indictments" have helped to clear the field of opposition candidates in some of the country's most high-profile electoral contests. At the same time, notoriously corrupt allies of the president have been left untouched. In fact, several of them have a clear run at electoral office because their only credible opponents have been struck from the ballot.

These recent moves cast a pall over Obasanjo's legacy as he prepares to leave office. They threaten to delegitimise the results of the April elections. Moreover, they have done inestimable damage to the credibility of his government's widely-heralded "war on corruption".

With the elections only weeks away, Atiku and others are challenging the legality of their disqualification before the courts. The court of appeal has ruled that Atiku and the others can contest the elections, but the electoral commission is challenging the matter in the supreme court. But even if the courts uphold Atiku's right to stand, by then it could be too late. Nigeria's electoral commission has already warned that adherence to such court judgements could derail the elections if the ballot papers have already been printed.

If the elections are indeed derailed, then the consequences for Nigeria will be grave. The vice-president has promised "anarchy" if he is not allowed to stand. The much-praised war on corruption that has collected many high-level scalps will appear to be little more than a political witch hunt. Once again, Nigerian voters could face the prospect of an election where the choices have already been made, and not by them.