WASHINGTON — The Senate impeachment trial of President Donald John Trump begins in earnest Tuesday with an expected outcome of acquittal but also the possibility of unpredictable twists and turns along the way.

The third impeachment trial of a U.S. president is the first in a divided Congress with entrenched partisan lines. It’s the first in the modern era to occur during an election year. And it’s the first in which the impeached president likely will react in tweets and public comments in real time during the proceedings.

The length of the trial could be as short as a couple of weeks, as senior officials in the White House hope, or it could run as long as the five weeks, or more, that it took the Senate to hear arguments and witnesses before acquitting President Bill Clinton in 1999.

The Senate must decide two issues: whether Trump abused his power and obstructed Congress by pressuring Ukraine to investigate his political rival Joe Biden, a Democratic presidential candidate by withholding U.S. aid, and then blocking the House investigation.

Scholars warn an impeachment trial is not like an ordinary trial.

The trial will be in session every day but Sunday until it is done.

It will operate under its own rules voted by the Senate. Senators will set their own individual standards of proof and rely on the evidence they choose. And they will convict or acquit as both the court and the jurors.

“This is political theater,” said Michael Gerhardt, an expert on impeachment at University of North Carolina Law School and CNN commentator. “It’s not so much about the law. It’s about the facts.”

Here are trial processes and moments to watch for over the next few weeks:

Rules of the road

A key fight will occur Tuesday over the rules of the trial — and especially whether new witnesses and documents can be introduced. The debate could be conducted in secret or public, depending on what the Senate decides.

In the Clinton trial, the rules were adopted unanimously after both the majority and minority leader agreed to them. But Senate Majority Leader Mitch McConnell (R-Ky.) and Senate Minority Leader Chuck Schumer (D-N.Y.) have not met this year about the process.

McConnell said he wants a straight-laced process based on the rules used in the Clinton trial and said he has the 51 votes to adopt it.

Schumer vowed to stage a floor fight to require the trial to include four unheard witnesses and three sets of unseen documents.

The trial cannot begin without rules, In the final rules resolution, the focus will be on how long each side gets for their arguments, what kind of evidence can be presented and if and when the Senate will vote on dismissal or the appearance of witnesses or their depositions.

The Clinton trial process

If the Senate adopts the Clinton trial rules, here is what will happen:

House managers will get 24 hours over three days to present their arguments to convict.

The White House will get 24 hours over three days to defend the president.

Then senators, who by their own rules cannot speak during the impeachment trial, will get 16 hours over two days to submit written questions for either the managers or defense team to the presiding officer, the Supreme Court chief justice, who will pose the questions verbally.

After the presentations and questions, the Senate will deliberate and decide whether to vote for dismissal of the charges, and if not, to vote to allow witnesses. Then it sets a date for the final vote on the articles of impeachment.

The umpire

Chief Justice John G. Roberts, who once likened his position to a baseball umpire, will serve as the presiding officer of the trial — a potentially difficult job.

“The Chief Justice will likely want to be seen as rising above partisanship, serving as an impartial arbiter of justice. To that end, Roberts will likely want to avoid letting the trial become a political circus,” Elizabeth Wydra, president of the Constitutional Accountability Center, a Washington think tank and law firm, wrote on her website.

Senate rules require the chief to make calls on questions of the “relevancy, materiality and redundancy of evidence.” Yet, the Senate can overrule him with 51 votes.

Rehnquist played a hands-off role in the Clinton impeachment. But Wydra suggested, “unlike Rehnquist, Roberts may simply be unable to avoid stepping into the fray and getting his robe dirty.”

Prosecution and defense

The House managers — seven Democratic lawmakers led by Rep. Adam Schiff (D-Calif.) — and the White House defense team — lawyers led by White House counsel Pat Cipollone — likely have learned the lesson of the Clinton trial: Don’t be boring.

When all 13 House managers arguing for the removal of Clinton spoke, some senators — deprived of cellphones, computers and any reading material not connected with the trial — nodded off at times as the same arguments were made again and again.

Schiff and his team include five experienced litigators and likely will use video clips of testimony, emails and texts from the impeachment inquiry.

Trump has added four well-known lawyers with extensive television experience to his team, including former Clinton investigation independent counsels Kenneth Starr, Robert Ray and Alan Dershowitz, an emeritus Harvard law professor.

Democrats have shown they can expand their arguments to include the Mueller report and other matters. And Starr and Dershowitz have been very outspoken about the Trump impeachment — raising the question of how far, loud and colorful each side will go.

Sen. Roy Blunt (R-Mo.), Senate Rules Committee chairman, said, “I suppose they can say anything they want to in their arguments.”

New evidence bubbling up

On the very first day of the Senate impeachment trial last Thursday, when Schiff read the articles of impeachment and Roberts swore in the senators, the Government Accountability Office issued a report saying the White House budget office violated the law in delaying Ukraine aid.

That followed the emergence of emails showing Trump ordered withholding of the aid shortly after talking with Ukraine President Volodymyr Zelensky and a letter by Trump’s personal lawyer Rudy Giuliani to Zelensky in May saying he was acting with the knowledge of Trump.

In recent days, Lev Parnas — an associate of Giuliani, who leads the effort to get Ukraine to investigate Biden — also has popped on TV news shows talking about the ousting and even surveillance of former U.S. Ambassador Marie Yovanovitch.

Sen. Chris Van Hollen (D-Md.) said he expects the surprises to keep coming, saying, “I think there’ll be a lot of stuff happening during the impeachment trial.”

The question is: Will the Senate include any of it in the trial?

Republicans on the edge

All eyes will be on the half dozen or so senators who represent states that are potential toss-ups in the November election as they cast votes on allowing witnesses and on impeachment.

Republicans will watch Susan Collins of Maine, Lisa Murkowski of Alaska, Lamar Alexander of Tennessee, Mitt Romney of Utah, Cory Gardner of Colorado and Joni Ernst of Iowa. Four of them could provide the votes Democrats will need to allow witnesses.

Captive candidates

For the first time, the Senate is trying an impeachment trial with mandatory attendance for its 100 members during an election year — requiring four Democratic presidential candidates to be in Washington during the two weeks leading up to the Feb. 3 Iowa caucuses.

“I’d rather be in Iowa,” Sen. Bernie Sanders (I-Vt.) said Thursday. How Sanders, Massachusetts Sen. Elizabeth Warren, Minnesota Sen. Amy Klobuchar and Colorado Sen. Michael Bennet juggle their constitutional responsibility and their political campaigns will be worth watching.

The battle for public opinion

The moment was icy but peaceful, overseen by wary U.S. Capitol police.

Anti-Trump protesters in black shirts and hoodies with slogans such as “Trump is Guilty” and pro-Trump demonstrators wearing yellow “Trump Defense” windbreakers turned up at the Dirksen Senate Office Building cafeteria for lunch at the same time last week.

In Washington and across the nation, activists on both sides have announced they will stage rallies and encourage phone calls to the offices of senators who show the slightest wavering from their party line.

Pro-impeachment groups, such as People for the American Way, MoveOn.org and others in the Impeach Now coalition will urge senators to vote to allow witnesses and documents in the trial. Anti-Impeachment groups such as FreedomWorks and Heritage Action want a fast dismissal or acquittal.

Both sides say they will move to the presidential election once the trial is done.

67

That is the most important number in the Senate trial.

There are 53 Republicans, 45 Democrats and two independents who vote with Democrats in the Senate.

And to convict and remove Trump from office, it takes 67 votes.

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