U.S. Supreme Court Chief Justice John Roberts (left), accompanied by Senate Sargeant at Arms Michael C. Stenger, arrives at the U.S. Capitol to be sworn in, as well as to swear in senators, before the start of the Senate Impeachment trial against President Donald Trump, in Washington, D.C., on Jan. 16 2020. (Photo: Diego M. Radzinschi/ALM)

A federal trial is set to begin in Washington, D.C., on Tuesday, but unlike other trials that regularly take place in the national's capital, the acquittal of the defendant, President Donald Trump, is all but guaranteed, and next to no one knows what the rules are.

Arguments will begin this week in an impeachment trial that presents a rare mix of legal and political proceedings. It's already off to a banner start, with Friday's announcement that former independent counsel Ken Starr and Harvard Law School's Alan Dershowitz will represent Trump.

The trial also presents a rare co-mingling of all three branches of the federal government, with Chief Justice John Roberts emerging from the privacy of the Supreme Court to preside over House prosecutors, a White House defense team, and senators playing both judge and jury.

The House filed its first brief for the trial Saturday, including recently revealed evidence about Trump personal attorney Rudy Giuliani's role in the Ukrainian-focused allegations, and urging the Senate to convict Trump and remove him from office or risk future presidents committing similar offenses.

"President Trump has betrayed the American people and the ideals on which the nation was founded," the House's seven impeachment managers argued in their brief. "Unless he is removed from office, he will continue to endanger our national security, jeopardize the integrity of our elections and undermine our core constitutional principles."

The House's brief Saturday also showed several attorneys are still working behind the scenes. A footnote lists House general counsel Douglas Letter and his staff as having helped on the brief, as well as Debevoise & Plimpton lawyers David O'Neil, Anna Moody and Laura O'Neill. The firm also is working on the House's separate lawsuit seeking documents on the failed attempt to add a citizenship question to the 2020 census.

Also named were House Intelligence committee investigators Daniel Goldman and Daniel Noble, and House Judiciary Committee lawyers Norm Eisen, Barry Berke, Joshua Matz and Sophia Brill.

The White House on Saturday also gave a preview of its case in a short answer to the summons issued by the articles of impeachment, attacking the process as "a brazen and unlawful attempt to overturn the results of the 2016 election and interfere with the 2020 election." The president's legal team filed its first formal brief Monday.

While the Senate proceeding is technically labeled a trial, it will veer far from the general trial procedures.

The U.S. Constitution offers few hints as to how each stage of the proceedings should be conducted, and only two other impeachment trials have been held in U.S. history, each a century apart.

Unlike the rules of procedure that guide trials in federal courts, Senate Majority Leader Mitch McConnell has said he's coordinating with White House counsel Pat Cipollone on the rules package for impeachment.

Critics have decried McConnell's strategy, comparing it to the jury and defense counsel working together before a trial. House Speaker Nancy Pelosi accused McConnell and Cipollone of being in "cahoots."

It's still uncertain as to whether witnesses will be allowed in the trial. Some Republican senators have said they'd vote to call some witnesses to testify, but the White House is opposing the move.

While Senate Republicans have reportedly been guided by the White House on impeachment, their Democratic counterparts have pulled in experts like University of Missouri's Frank Bowman and Harvard's Laurence Tribe to learn the ins and outs of impeachment.

Much has also been made of Roberts' role in the trial. But, beyond making procedural rulings when required, the chief justice is unlikely to take a position that would ruffle many feathers or make a dramatic difference during the trial.

On Saturday, a source close to Trump's legal team said that the plan is currently for White House counsel Pat Cipollone to deliver the opening argument for the trial, and Trump's personal lawyer Jay Sekulow will then give "an overview on the entire process." But the source said that could change.

Other attorneys on the team will have "discrete functions" that they will handle, but the source said they did not know at that time how that would "play out."

Starr, the former independent counsel charged with investigating former President Bill Clinton, is expected to touch on the obstruction of Congress charge against Trump. He had argued in the Clinton impeachment that the president committed obstruction, and that it was an impeachable charge.

"Bill Clinton was asserting executive privilege over private conduct," the source close to Trump's legal team said Saturday. "This is markedly different, and I think that's a clean distinction and clean draw." The source added that they think they'll be "very pleased" with Starr's presentation in the trial.

Two of the president's lawyers are also linked to Jeffrey Epstein: Starr was on his defense team in 2007, as Epstein struck a plea bargain with then-U.S. Attorney Alex Acosta that helped him avoid federal charges of sex trafficking. Dershowitz has also faced allegations of having sex with victims trafficked by Epstein, which he has repeatedly denied and filed lawsuits in response to the claims.

Besides Starr and Dershowitz, Trump's legal defense team features Cipollone, the unassuming White House counsel who has led the president's defense during the House impeachment inquiry. He caught flack from attorneys on both sides of the aisle during the House proceedings for his approach, which included demanding due process rights that generally aren't afforded until trial.

Sekulow, who also worked on the Mueller probe, will also be on the president's defense team. Robert Ray, who took over for Starr as independent counsel in the 1990s, Florida attorney Jane Raskin and former Florida state attorney general Pam Bondi, who has helped the White House on impeachment messaging, and Eric Herschmann of Kasowitz, Benson, Torres also are involved.

But the elephant in the chamber is likely to be Giuliani, Trump's lawyer who has figured prominently in the Ukraine matters.

Former Trump lawyer Jay Goldberg, who represented the president in his divorces, said last week that he had warned Trump three times against hiring Giuliani. He said he believed Giuliani's trips to Ukraine were unnecessary and ultimately caused more issues for the president.

"As a prosecutor you tend to overdo it. He wanted to try and wrap it up," Goldberg said of Giuliani's efforts in Ukraine. "He's not a seasoned lawyer who knows when to call it quits."

Goldberg's been complimentary of Sekulow in the past. He also had praise for Cipollone, calling him "erudite and articulate," although Goldberg said he hadn't worked alongside him.