You call this a trial? When it comes to impeachment, not so much.

The Senate impeachment trial of President Donald Trump is full of references from both sides to evidence, due process, the Constitution, and of course, fairness.

But it’s important to realize that this is not a trial in the sense of the TV classic “Law & Order,” but a hybrid – part courtroom presided over by the chief justice, and much more a political arena.

Senators sit as jurors and judges. Hardly sequestered, they run to the cameras during breaks to hold dueling press conferences. And they set their own rules, which they can also change.

“An impeachment trial was never meant to be a real trial,” says Ray Smock, former historian for the U.S. House of Representatives. It is a constitutional process taking place in a political environment, a “power play” between the executive and legislative branches, he says.

“Once power is involved,” he adds, “you can’t escape political machinations.”

Nowhere is this judicial and political mixture more consequential than on the issue of witnesses.

Democrats desperately want to call them, as well as subpoena documents, in order to bolster their allegation that the president shook down a foreign leader – withholding military aid and a White House meeting, for his own political benefit. They pound the point that a trial has witnesses, and to be fair, this trial must also have them.

But the Constitution is silent on this question – and pretty skimpy on instructions for impeachment generally.

“Because this is not a trial, there is no guarantee of witnesses,” says Mr. Smock.

Senators have agreed to vote on whether to have witnesses, probably on Friday. They wanted to wait for both sides to finish their presentations, which now have concluded, and for senators to ask questions of the two teams on Wednesday and Thursday. In a chamber controlled by 53 Republicans, Democrats need four Republicans to split from the pack if they want witnesses.

Just days ago, such prospects seemed to be fading. But like lightning, new information fell from the sky, electrifying Washington and recharging Democrats’ hopes. On Sunday, The New York Times reported that in his forthcoming book, former national security adviser John Bolton writes that the president told him that military aid to Ukraine was contingent on that government investigating Democrats, including former Vice President Joe Biden and his son, Hunter. Mr. Trump denies having said that to Mr. Bolton.

The president’s defense team has steadily maintained there was no link between the aid and the investigations.

The manuscript leak prompted Republican Sen. Mitt Romney of Utah to tell reporters Monday that it’s “increasingly likely” that more Republicans will join those who want to hear from Mr. Bolton – a top witness choice for Democrats because of his first-hand knowledge of the president’s stated intentions. Sen. Susan Collins (R-Maine) also said in a statement that reports about the book “strengthen the case for witnesses” and have prompted “conversations” among her colleagues. Other Republicans say they want to see the manuscript.

Witness quid pro quo?

The witness issue is fraught with risk for both sides. First, there’s no telling where witnesses might go, even if they are deposed first, as agreed on by the senators. Second, if the Senate goes down this road, the GOP majority could vote for witnesses politically harmful to the Democrats.

One possibility floated by Republicans is a witness deal, perhaps John Bolton for the Democrats and Hunter Biden for the president. But Democrats shot down that idea, saying he wasn’t a material witness (not to mention the political damage that calling him might heap on his father’s presidential campaign).

Absent a deal, and if enough Republicans vote for witnesses, the president could exert executive privilege to block certain witnesses from testifying. The White House is warning of a long, litigious battle if witnesses are subpoenaed.

With new information dribbling out as the days wear on, it’s in the president’s interest to wrap this trial up quickly, preferably before the State of the Union address next week.

Meanwhile, four Democratic presidential candidates are locked inside the Senate just as the Iowa caucuses and New Hampshire primary are about to break.

Yet the court-battle scenario is “a lot of chaff being put in the air,” counters Democratic Sen. Chris Coons of Delaware. The Constitution empowers the Senate as the sole body to try the president. If it voted to issue a subpoena, it would be signed by the presiding officer of the impeachment trial – Chief Justice John Roberts.

“I would argue [that] is not reviewable” in the courts, Senator Coons says.

Some Republican senators argue that witnesses are not necessary because the charges against the president – abuse of power and obstruction of Congress – do not rise to the level of impeachment.

That’s what was argued Monday by Mr. Trump’s celebrity defense attorneys, constitutional scholar Alan Dershowitz and Kenneth Starr, the independent counsel whose investigation sparked the impeachment of President Bill Clinton. The former president was acquitted by the Senate 20 years ago.

‘Age of impeachment’

With professorial flourish, Mr. Dershowitz rewound history to argue that the constitutional criteria for impeaching, convicting, and removing a president – “treason, bribery, or other high crimes and misdemeanors” – were intended to mean offenses that were “criminal-like.” Were that not the case, he argued, a long list of presidents from George Washington to George H.W. Bush might have been impeached for “abuse of power.”

The president’s lawyers maintain he committed no crime, that there was no “quid pro quo,” and that his temporary hold on military aid was related to corruption and burden-sharing.

Mr. Starr warned that America is living in an “age of impeachment,” with the tool “weaponized” against presidents from Nixon, to Clinton, and now Mr. Trump. Indeed, the House impeachment vote in December fell almost entirely along party lines, with only three Democrats siding with Republicans in opposition, and no Republicans voting to impeach. Mr. Starr pointed to Britain, from which the United States inherited the impeachment process. Parliament stopped using impeachment after 1868 because the process failed to meet the modern standards of “fairness,” he said.

Fairness is an issue constantly raised by both sides in this trial. It dominated the first full day of deliberations, when senators debated the rules, and continues still. Were this a courtroom trial, with established rules and procedures, there would be far fewer cries of outrage, say government and legal experts.

Bernadette Meyler, a constitutional scholar at Stanford University in Palo Alto, California, points out that even though witnesses are not a constitutional necessity in this trial, it’s what people generally expect – “a moral fairness” she says, if not a “legal requirement.” Two-thirds of Americans (66%) believe the Senate should call in new witnesses, according to an ABC-Washington Post poll released at the end of last week.

“Fairness is what the Senate decides,” said former Sen. Jeff Flake (R-Arizona), who dropped by the trial last week. Unless Republicans break ranks, those decisions will be determined by the GOP majority, with both sides – and the nation – having to reckon with the consequences.