A key witness in the federal criminal trial of Roger J. Stone Jr. testified on Thursday that Mr. Stone had misrepresented him to congressional investigators in 2017 as his intermediary with WikiLeaks even though he had repeatedly urged Mr. Stone to “tell the truth.”

The testimony of Randy Credico, a New York radio host, seemed to bolster the prosecution’s contention that Mr. Stone, a former Trump campaign aide, had deliberately lied in his testimony to the House Intelligence Committee in September 2017. At the time, the committee was investigating Russian interference in the 2016 presidential election, including the role of WikiLeaks.

But the radio host’s credibility remains an issue in Mr. Stone’s trial, now in its third day in a federal courthouse in Washington. Mr. Stone, a 40-year friend of President Trump, has been charged with obstruction of justice, making false statements and tampering with a witness in what prosecutors claim was an effort to shield Mr. Trump and his campaign.

Prosecutors warned jurors this week that Mr. Credico was a less than perfect witness. On the stand, he acknowledged that at points in late 2016, he had exaggerated his relationship with Julian Assange, the founder of WikiLeaks, to Mr. Stone.

At the time, Mr. Stone was trying to reach Mr. Assange to obtain information about tens of thousands of emails that Russian intelligence operatives had stolen from Democratic computers and funneled to WikiLeaks in an effort to hurt Hillary Clinton, Mr. Trump’s political opponent.