One of former Prime Minister Benjamin Netanyahu’s closest advisers testified against him Monday in a long-running corruption case that could decide the political fate of Israel’s longest-serving leader.

The trial has become one of the starkest symbols of the reversal of fortunes for Mr. Netanyahu, who led the country for 15 years until he failed to form a government this spring. Prosecutors say Mr. Netanyahu accepted expensive gifts from wealthy businessmen in exchange for official favors and offered two media moguls regulatory and financial benefits in exchange for positive press coverage.