Mr Nadler, whose committee has the power to conduct impeachment hearings, also said it was "very clear" that the president had obstructed justice.
But Mr Nadler told ABC News it was too early to discuss removing the president from office.
President Trump dismissed Mr Nadler's probe as a "political hoax", but added: "I co-operate all the time with everybody."
Democrats, who took control of the House of Representatives in January, have vowed to open investigations into the president and White House.
- ۹۷/۱۲/۱۴