Witness Tampering

Trump– the Chief Executive of the United States– discouraged cooperation by witnesses in his own government’s investigations. He used mob-boss language and even floated pardons as inducements.

The public and private witness meddling began in force after the firing of Comey
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When Michael Flynn began cooperating with the investigation, WH Counsel John Dowd left a voicemail pressuring him not to cooperate.
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Trump called Manafort “strong” and didn’t rule out a pardon during his trial. He praised him for not cooperating.
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Trump first told Cohen to “stay strong” and had his WH counsel discuss a pardon with him. Then he called him a “rat” and suggested he’d indict family members.
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Mueller explicitly stated that even public witness tampering is illegal and of great harm to our justice system.

Discouraging cooperation by witnesses was just one of the ways Trump obstructed justice. See Obstructive Acts for other acts of obstruction.