PRESIDENT BUSH'S lack of credibility with African leaders is hurting diplomatic efforts to bring an end to political and economic repression in Zimbabwe. The leaders are dragging their feet on getting the country's dictator, Robert G. Mugabe, to negotiate with opposition leaders on a transitional government and new elections. Bush has little standing to make the major players in Africa do otherwise. But that shouldn't be an excuse for him not to invest more energy on the crisis in Zimbabwe, where conditions are rapidly deteriorating. Unprecedented food and oil shortages, triple-digit inflation, and violent government crackdowns on political dissent have caused residents to flee the country. Dozens of people have been killed in state-orchestrated violence, and thousands have been beaten, jailed, or tortured for their views. By some estimates, more than 3.5 million Zimbabweans have fled to South Africa, Botswana, or Britain.