Mr. Trump forced out as ambassador to Ukraine in May. Ms. Daum represents Kurt D. Volker, who resigned last month as the special envoy to Ukraine, and Philip T. Reeker, the acting assistant secretary of state for Europe and Eurasia.
Other lawyers have deep backgrounds in intelligence or foreign policy, including John Bellinger, a legal adviser to the National Security Council and the State Department under President George W. Bush, and Lee S. Wolosky, a national security official under both Mr. Bush and President Bill Clinton.
Mr. Bellinger represents William B. Taylor Jr., the top American diplomat in Ukraine who on Tuesday provided the most damning testimony to date against the president. He also represents Michael McKinley, who quit his senior State Department post after the White House forced out Ms. Yovanovitch.
Mr. Wolosky, who represents Fiona Hill, a former Russia expert on Mr. Trump’s National Security Council, wound up in a standoff with the White House Counsel’s Office over his client’s testimony.
Three days before Ms. Hill was to testify, two lawyers from the counsel’s office argued to him in a telephone conversation that the discussions she was privy to were protected by executive privilege.