ALEXANDRIA, Va.—An attorney for Paul Manafort, the former Trump campaign chairman, attacked the credibility of Mr. Manafort’s longtime associate Richard Gates on Tuesday, pressing him on lies he admitted telling Mr. Manafort, prosecutors and his wife.

Over two hours of questioning during Mr. Manafort’s trial on tax- and bank-fraud charges, Mr. Gates acknowledged he had an extramarital affair, represented Mr. Manafort’s investments as his own, created a fraudulent document for another business partner, was the subject of an insider-trading investigation at the Securities and Exchange Commission and embezzled hundreds of thousands of dollars from Mr. Manafort.

At moments, Mr. Gates was combative with Kevin Downing, Mr. Manafort’s lawyer, about the extent of his betrayals and false statements, including those to the Federal Bureau of Investigation.

Mr. Gates also expressed remorse and said he was trying to turn his life around.

“I was living beyond my means,” he said, explaining why he stole from his boss, later adding that “I am here to take responsibility. Mr. Manafort had the same path. I am here.”