(CNN) Donald Trump has appeared "distracted" by the impeachment trial that begins on Tuesday, according to a source close to the White House who speaks to the President regularly.

"Why are they doing this to me," the source quoted Trump as saying repeatedly, telling people around him this weekend at Mar-a-Lago that he "can't understand why he is impeached."

Trump has been telling associates and allies around him that he wanted a "high profile" legal team that can perform on television, the source said. It's simply who Trump is, the source continued, adding Trump loves having people who are on television working for him.

This in part may explain why Kenneth Starr and Alan Dershowitz were added to the legal team representing the President.

Both sides make their case

The Democrats filed to the Senate their trial brief, a summary explaining why the House passed two articles of impeachment last month charging Trump with abuse of power and obstruction of Congress.

"President Trump's conduct is the Framers' worst nightmare," the managers wrote in the brief.

"President Donald J. Trump used his official powers to pressure a foreign government to interfere in a United States election for his personal political gain, and then attempted to cover up his scheme by obstructing Congress's investigation into his misconduct," the managers wrote in the brief. "The Constitution provides a remedy when the President commits such serious abuses of his office: impeachment and removal. The Senate must use that remedy now to safeguard the 2020 U.S. election, protect our constitutional form of government, and eliminate the threat that the President poses to America's national security."

Trump's legal team filed its formal response Saturday evening to the Senate summons of the President, offering the first glimpse into what will ultimately be the White House's impeachment defense.

The response argued both substantively, against the charges in the articles, and procedurally, against the House's impeachment inquiry.

"President Trump categorically and unequivocally denies each and every allegation in both articles of impeachment," the document reads

The legal team argues that the first article of impeachment, abuse of power, "alleges no crime at all, let alone 'high crimes and Misdemeanors,' as required by the Constitution." The team cited Ukrainian President Volodymyr Zelensky's repeated denials that he felt any pressure from Trump as evidence that Trump did not abuse his power during the July 25 phone call.

The team pointed to the fact that the President released transcripts of both the July 25 phone call and an earlier one on April 21 to argue the conversations were "perfectly legal, completely appropriate and taken in furtherance of our national interest."

Seasoned lawyers

Starr, the hard-charging prosecutor whose work led to President Bill Clinton's impeachment, and Dershowitz, the constitutional lawyer, will join Robert Ray, Starr's successor at the Office of Independent Counsel during the Clinton administration, on the defense team, White House press secretary Stephanie Grisham said earlier in a statement.

The additions of Starr and Ray to Trump's legal team happened over the last three to four weeks, according to a source familiar with the legal team's thinking.

The legal team was aware of Trump's previous comments about Starr, when in 1999 the President called Starr a lunatic , before they broached the idea with him, but they didn't think it was a big deal.

In the end, "the President wanted these guys out there," the source said.

It is unclear if the President was aware of Starr making recent comments on Fox News about compelling evidence against him in reaction to the testimony of Gordon Sondland, the United States Ambassador to the European Union.

Dershowitz was pressured by White House to join legal team, source says

The President was especially fixated on having controversial defense attorney Dershowitz on the legal team. But Dershowitz has been telling his own associates he didn't want to participate in the President's trial, the source who is familiar with these conversations said. White House officials have applied a lot of pressure over the last several weeks to convince Dershowitz to join the team, sources familiar with the attorney's appointment said.

Discussions about bringing in Dershowitz happened over a longer period of time, as CNN has previously reported, and he was also speaking directly to the President about his views on the impeachment case unlike Starr and Ray.

"Alan has been more engaged" behind the scenes, the source said.

The three seasoned lawyers are expected to join a legal team headed by White House counsel Pat Cipollone and outside attorney Jay Sekulow, who are still expected to deliver statements on the President's behalf on the Senate floor.

Sekulow has a long history working with Starr, going back to when Starr was solicitor general and Sekulow argued to the Supreme Court. Starr was a partner Cipollone's at Kirkland and Ellis.

When Sekulow and Cipollone were trying to determine who else should argue on the floor, it seemed like Starr and his former deputy would be natural fits for their combination of legal chops, television experience and notoriety.

The source says they're well respected lawyers with experience on impeachment and the TV experience "doesn't hurt."

Former Florida Attorney General Pam Bondi, Trump's longtime personal counsel Jane Raskin and attorney Eric Herschmann will also supplement the President's impeachment legal team, Grisham said. All are expected to have speaking roles, people familiar with the matter told CNN.

Trump's response to his legal team

When the President was presented with his final team by his lawyers, he was pleased with it and gave the ultimate sign off, but he had also previously been looped in on the idea of Starr and the others.

Also, one of the reasons the President is beefing up his legal team is to break up the monotony of the floor arguments over the 24 hours they are expected to have to make their case, according to a person familiar.

The legal team has been strategizing on how to make what are typically dry legal arguments more interesting by changing up who is delivering the message, in part to appease the President who has said privately he wants a show fit for television.