The source tells TIME that White House counsel Pat Cipollone and Donald Trump's personal lawyer Jay Sekulow will take the lead on the defense.
The team will also include Pam Bondi, the former Florida attorney general and a Trump ally, according to a person who spoke to The Associated Press on condition of anonymity because the person wasn't authorized to speak on the record.
Opening arguments are to begin next Tuesday after the Martin Luther King Jr. holiday. He's facing increasing pressure to call witnesses during the trial, as new information continues to emerge about President Trump's effort to pressure Ukraine to investigate his political rival, Joe Biden, ahead of this year's presidential election. That would be far shorter than the trial of President Bill Clinton, in 1999, or the first one, of President Andrew Johnson, in 1868.
He was removed as president of Baylor University and then resigned as chancellor of the school in the wake of a review critical of the university's handling of sexual assault allegations against football players. Critics of Starr at the time accused him of turning a blind eye to sexual violence on his campus after pursuing Clinton for a sexual relationship.
USA media outlets widely reported the addition of both men to Mr Trump's legal team on Friday along with a third, Robert Ray, who helped investigate Mr Clinton's presidency, including the so-called Whitewater scandal.
Mr Dershowitz has been a well-known figure in United States legal circles for decades.
The US Congress has opened the impeachment of US President Donald Trump, with House Democrats reading the formal charges from the well of the US Senate.
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It adds an additional "what you have" layer to the standard "who you are" (username), and "what you know" (password) level of security.
Mr Ray succeeded Mr Starr as the independent counsel.
Democrat Adam Schiff, who heads a team of seven House members who will serve as prosecutors, appeared on the Senate floor on Thursday to read the two charges passed by the House. Schiff is a former federal prosecutor in Los Angeles.
Still to be determined in the trial is whether the Senate will vote to allow witness testimony and new evidence or whether senators will decide the case as Republican Majority Leader Mitch McConnell has suggested using only the material amassed by House investigators.
Trump faces two charges after the House voted to impeach him last month.
Romney said he wants to hear from John Bolton, the former national security adviser at the White House, who others have said raised alarms about the alternative foreign policy toward Ukraine being run by Giuliani.
Democrats will need to win the support of some moderate Senate Republicans if they hope to get a majority in the chamber to permit the calling of witnesses.
The trial in the Republican-controlled Senate is unlikely to lead to Trump's ouster, as no Republican senators have voiced support for doing so. We'll have more on the impeachment trial after headlines.
Kenneth Starr, seen in 2014, served as independent counsel, providing findings that helped lead to the impeachment of President Bill Clinton in 1998.