Which political leader supported a policy of constructive engagement, which
desmond tutu criticized in the 1980s?

Ronald Reagan

Explanation:

During the beggining of the 1980s, Reagan had a policy called constructive engagement that claimed that the U.S. could better the situation of people oppressed by apartheid, by maintaining good relationships with the government in South Africa.  

Archbishop Desmond Tutu, who supported the imposition of international economic sanctions as the answer to a peaceful transition, vigorously opposed Reagan´s policy.