Ecuador’s decision to pull the plug on Julian Assange’s internet connection has highlighted the isolation of WikiLeaks, the organization he founded to expose the inner workings of governments and other powerful institutions.

Some former allies and observers say that after four years confined to the Ecuadorean Embassy in London, Mr. Assange is alienating former supporters and undermining WikiLeaks’ relevance. They cite a series of leaks that they say supported Russian efforts to disrupt the U.S. election and carelessly promoted...