WASHINGTON — Gregory B. Craig, one of Washington’s most powerful Democratic lawyers, testified in his own defense in a federal felony trial on Wednesday, denying passionately that he had deceived federal investigators six years ago about whether he had aided a public relations campaign by Ukraine’s president to repair his battered image in the United States.

“I did not lie,” Mr. Craig said, looking directly at the jury in the most dramatic testimony of his nearly two-week trial in the federal courthouse in Washington. “I did not withhold any information.”

The unusual case against Mr. Craig, 74, turns on whether he misrepresented his contacts with journalists over a 2012 report by his law firm examining whether a political rival of Viktor F. Yanukovych, then Ukraine’s president, had received a fair trial on corruption charges.

Mr. Yanukovych was hoping the report, by the internationally known law firm Skadden, Arps, Slate, Meagher & Flom, would help him blunt criticism that threatened Ukraine’s hopes of joining the European Union. Federal prosecutors contend that Mr. Craig, then a Skadden lawyer, lied about his role in the public relations campaign to safeguard his reputation and career prospects, as well as those of his colleagues.