(CNN) When the US Senate moves forward with an impeachment trial of soon-to-be former President Donald Trump, senators would have full control over how they run it, based on history and Supreme Court precedent.
Trump, who desperately fought his first impeachment proceedings and has remained defiant through his second, could not appeal the Senate's action.
The Supreme Court affirmed in a 1993 case that impeachment is the domain of the two chambers of Congress, with the US House having the right to accuse and the Senate the "sole" power to try the accusation.
Then-Chief Justice William Rehnquist wrote that the Constitution leaves no room for judges to intervene in Congress' business. To rule otherwise, Rehnquist said, would conflict with the framers' intentions and expose the country to "months, or perhaps years, of chaos."
Still, an array of questions has emerged since the US House of Representatives on January 13 impeached Trump, opening the door to a Senate trial, for inciting the violent resurrection at the Capitol on January 6.
Practical considerations hover along with substantive issues. House Speaker Nancy Pelosi has not yet set a date for delivery of the articles of impeachment to the Senate, and senators would be organizing a trial just as they take up the agenda of new President Joe Biden.
No president has been tried after leaving office. In earlier centuries, however, two other officials were impeached and subjected to some Senate action after they resigned. Among the basic questions spurred by a groundbreaking trial for an ex-president is whether the Chief Justice John Roberts would preside, as he did at Trump's first impeachment trial one year ago.
Many legal scholars believe a former president can be subject to trial, and even some who differ, recognize that the 1993 precedent, in which Mississippi federal judge Walter Nixon unsuccessfully challenged Senate trial procedures in his impeachment case, case could thwart a Trump appeal to federal courts.
Yale University law professor Akhil Reed Amar said constitutional history and precedent allow the Senate to try former officials, including the President.
"It would be absurd if you could escape by resigning one step ahead of the gavel," said Amar, author of "America's Unwritten Constitution."
In 1876, the Senate tried William Belknap, who was war secretary in the administration of Ulysses S. Grant. Belknap resigned just as the US House was voting to impeach him.
He was charged with "criminally disregarding his duty as Secretary of War and basely prostituting his high office to his lust for private gain."
The Senate determined that the former secretary could still be tried. After a lengthy Senate trial with dozens of witnesses, Belknap was acquitted.
University of Missouri law professor Frank Bowman, author of "High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump," said the Constitution provides for removal, as well as "disqualification" from holding office, because the framers were concerned about future dangerous conduct.
"The framers were worried, principally, about getting a president out of office," Bowman said, "but they were also worried about the rise of a demagogue."
Bowman and Amar say the Walter Nixon case, along with the text and structure of the Constitution, would prevent Trump from drawing federal judges into an impeachment dispute.
Ross Garber, who teaches at Tulane Law School, asserts that the Senate may try only a sitting president but nonetheless agrees it would be difficult for Trump to find a court that would hear his appeal.
"I think the reasoning of Nixon (case) could be a problem for any Trump litigation effort," he said, adding that "it is very unlikely the Supreme Court would stop the Senate in its tracks in a direct Trump challenge to its jurisdiction."
Garber added: "Strategically, Trump is likely better off not litigating but instead appealing directly to the senators."
The Constitution dictates that "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
The process for impeaching a federal judge is the same as impeaching a president. Judge Nixon was impeached and convicted in 1989, after an earlier criminal conviction for lying to a grand jury about his efforts on behalf of a defendant whose father had given his money. Federal judges are appointed for life, and impeachment is the only way to remove them from their post and judicial salary.
After his Senate conviction, Nixon's lawyers challenged in federal courts the Senate's use of a committee to take evidence, saying that violated the full Senate's constitutional responsibility to "try all impeachments."
The Supreme Court unanimously rejected Nixon's claim. Chief Justice Rehnquist was among seven of the nine judges who found that the case presented a "political" question beyond review by judges.
In his opinion for the high court, Rehnquist explored the 18th Century constitutional debates over where the impeachment power should reside. "According to Alexander Hamilton," Rehnquist wrote in Nixon v. United States, "The Senate was the 'most fit depositary of this important trust' because its Members are representatives of the people."
Justice David Souter, who agreed that the case presented a "political question," wrote separately to suggest an exception for maneuvers that threatened the integrity of the result, "say, upon a coin toss." But no other justice joined his concurring opinion.
Nearly six years later, Rehnquist presided over the impeachment trial of President Bill Clinton, just as Chief Justice Roberts did for Trump's first trial in 2020. The Constitution states, "When the President of the United States is impeached, the Chief Justice shall preside," rather than any senator or the vice president, who under the Constitution serves as the Senate president.
But if the trial is of an ex-president, the question of who presides is an open one.
Amar is among those who believe Chief Justice Roberts would not take the chair high on the Senate dais this time.
He said the framers' rationale for requiring the chief justice to preside in trials of a president stems from the natural conflict of interest of a presiding vice president. If the Senate convicted and removed the President from office, the vice president would succeed him.
The framers believed the chief justice should preside, Amar said, "precisely because the vice president is next in line."
But Bowman said even after a president has left office, a chief justice could still oversee a Senate trial.
"Any prudent organizer of the trial would insist that the chief preside," he said, adding that the presence of the chief justice of the United State would lend "legitimacy" to the proceeding. "Now, he will have his own views on the matter. But the best read (of the Constitution), and the most prudent read, is that he would preside."
Asked what Chief Justice Roberts thought, a spokeswoman said he had no comment.