President Donald Trump is bringing on private lawyers, including Kenneth Starr and Alan Dershowitz, to his legal team to defend him as his impeachment trial is set to begin next week in the U.S. Senate.

Starr, a former U.S. appeals court judge, and Dershowitz, a Harvard Law School professor emeritus, have defended Trump in numerous appearances on Fox News. Trump is also adding Robert Ray, a former federal prosecutor who succeeded Starr as independent counsel, according to a source connected to Trump's legal team. Starr recently called Trump's impeachment a "very ugly chapter in our constitutional history."

Dershowitz on Friday said on Twitter that he will present oral arguments on the constitutional grounds against impeachment. Proceedings are set to begin Tuesday afternoon. On Thursday, Chief Justice John Roberts Jr. delivered the oath to U.S. senators requiring them to render "impartial justice."