“Please ask Assange for any State or HRC e-mail from August 10 to August 30–particularly on August 20, 2011,” Mr. Stone wrote to Randy Credico, a New York radio personality who had interviewed Mr. Assange several weeks earlier. Mr. Stone, a longtime confidant of Mr. Trump, had no formal role in his campaign at the time.

Mr. Credico initially responded to Mr. Stone that what he was requesting would be on WikiLeaks’ website if it existed, according to an email reviewed by the Journal. Mr. Stone, the emails show, replied: “Why do we assume WikiLeaks has released everything they have ???”

In another email, Mr. Credico then asked Mr. Stone to give him a “little bit of time,” saying he thought Mr. Assange might appear on his radio show the next day. A few hours later, Mr. Credico wrote: “That batch probably coming out in the next drop…I can’t ask them favors every other day .I asked one of his lawyers…they have major legal headaches riggt now..relax.”