It was an opportunity that Representative Chris Collins, a Republican from Western New York, believed was too good to pass up. He was so confident of his investment in an Australian drug company that he hawked it to his children, his staffers, even other members of Congress.

“One of the things was, ‘Who in Congress have you talked with about Innate?’” Mr. Collins said of the company, Innate Immunotherapeutics, in a 2017 interview. “The bigger question would be: Who haven’t I talked to?”

A version of that question became Mr. Collins’s undoing. On Tuesday, he pleaded guilty to conspiracy to commit securities fraud and lying to federal investigators, after admitting passing private information about Innate to his son to help him avoid financial losses. Mr. Collins faces a prison sentence of up to 10 years; his son and the father of his son’s fiancée are expected to plead guilty to related charges later this week.

“I regret these actions beyond anything,” Mr. Collins, 69, told the judge, Vernon S. Broderick, in Federal District Court in Manhattan.