Thomas J. Barrack Jr., the chairman of President Trump’s inaugural committee, is under pressure from an activist investor to resign as chief executive of Colony Capital, a real estate investment firm he founded.

Blackwells Capital sent a searing letter to Colony on Tuesday, demanding that Mr. Barrack be replaced after Colony’s stock price plunged over the last three years. The investment firm also cited two federal investigations of the inaugural committee, calling them a distraction for Mr. Barrack.