Alan Dershowitz, the constitutional lawyer, and Robert Ray, Starr's successor to the Office of Independent Counsel under the Clinton administration, are also joining the team, the people said.
The three are expected to join a legal team led by White House Attorney Pat Cipollone and outside lawyer Jay Sekulow, who are still expected to make statements on behalf of the president on the Senate floor. Former Florida State Attorney Pam Bondi and Trump's longtime personal attorney Jane Raskin will also complement the president's impeachment legal team, a person familiar with the matter said.
A spokesman for Trump's legal team said Dershowitz will present oral arguments during the Senate trial.
"He is participating in this lawsuit to defend constitutional integrity and prevent the establishment of a dangerous constitutional precedent," said the legal team, which noted that Dershowitz opposed Clinton's persecution and voted for his wife, Hillary Clinton, in the presidential election in 201
Starr and Ray are expected to play a constitutional and historical role during the trial to support Cipollone and Sekulow.
Bondi has worked in the White House as an adviser during the case. Raskin is a white collar lawyer who, along with her husband, worked for Trump during the Mueller investigation.
A decision to allow House GOP allies to join the defense is still fluid, people say.
Officials have been hesitant to talk about who they are adding to the legal team after a failed attempt to hire Trey Gowdy last fall. Gowdy became the new face of Trump's imitation defense team just hours before Trump said federal lobbying rules wouldn't let him begin until January. He still has to talk to Gowdy privately about the situation.
This story has been updated to include additional reporting and background.