WASHINGTON — President Trump reacted angrily on Thursday to the relatively short prison sentence handed down to his longtime personal attorney and fixer, Michael D. Cohen, and said Mr. Cohen failed him as his lawyer because he should have known about campaign finance law.

“I never directed Michael Cohen to break the law,” Mr. Trump said in a series of Twitter posts. “He was a lawyer and he is supposed to know the law.”

Mr. Cohen was sentenced to three years in prison on Wednesday, related to two separate cases — a reduced sentence that reflected the information he shared with federal prosecutors, including details that implicated Mr. Trump in criminal activity. Mr. Cohen had faced a much lengthier sentence for his crimes of tax evasion, bank fraud, campaign finance violations and lying to Congress.

Mr. Trump said that his former lawyer had pleaded guilty to the charges in order to embarrass him and to get a reduced prison term. The president also dismissed the seriousness of lying to investigators — a federal crime that could lead to up to five years in prison. Referring to the special counsel, he said, “They want to scare everybody into making up stories that are not true by catching them in the smallest of misstatements. Sad!”