“He was the access point to Julian Assange,” Mr. Bannon said, who was dressed in black, spoke in clipped tones and frowned almost continually. “It was natural that I would reach out to him.”

In fact, Mr. Stone, a former campaign adviser, had no real ties to WikiLeaks, although he repeatedly tried to dispatch intermediaries to obtain the stolen emails or information about when they would be released.

He is charged with lying to congressional investigators about the identity of his go-between, the results of his efforts and his communications with Trump campaign officials. Prosecutors have said Mr. Stone hid hundreds of text messages and emails from the House committee because the truth would have embarrassed Mr. Trump and the campaign.

Mr. Credico, perhaps the most important witness in the prosecutors’ case, testified Friday that Mr. Stone pressured him not to cooperate with congressional investigators because his account of events would have exposed Mr. Stone’s lies. When he wavered, he said, Mr. Stone threatened him, his friend and his dog.

“You are a rat, a stoolie. You backstab your friends,” Mr. Stone emailed Mr. Credico in April 2018. “My lawyers are dying to rip you to shreds. I am going to take that dog away from you.”

Mr. Stone told the House committee that in summer 2016, Mr. Credico had confirmed for him that WikiLeaks possessed Mrs. Clinton’s emails. But Mr. Credico testified that although he is a close friend of Margaret Ratner Kunstler, a lawyer who provided legal counsel to Mr. Assange, he never provided information from WikiLeaks to Mr. Stone.

He said that he passed on one request to Mrs. Kunstler from Mr. Stone, asking for specific emails involving Mrs. Clinton, but in September, not earlier that summer. Both he and Mrs. Kunstler testified that she never answered.