The usually dignified US senate could see rising tensions along party lines as Donald Trump’s impeachment begins.

The upper chamber of US congress will see rock star legal teams cram the well just a few feet from each other and US chief justice John Roberts, as proceedings get under way.

Democrats in the US house of representatives impeached the president last month on two charges: abuse of power by withholding US military aid to Ukraine as he pressed that country to investigate Mr Trump’s Democratic rival Joe Biden, and obstruction of congress by refusing to comply with their investigation.

READ MORE: Trump on trial: The bloodying of the US president

This is what to watch when the US president’s trial begins at around 1pm local time (6pm GMT) on Tuesday:

Mitch McConnellMitch McConnell (Senate television/AP)

– Ground rules

The senate opens with a debate on the structure and rules of the proceedings. Senate majority leader Mitch McConnell is proposing a condensed, two-day calendar for opening arguments on the articles of impeachment passed by the US house on December 18.

Mr McConnell’s ground rules are outlined in a four-page resolution that must be voted on as one of the first orders of business. It pushes back any votes on witnesses until later in the process, rather than up front, as Democrats had demanded.

But Mr McConnell’s plan on witnesses lines up with the organising resolution which set the structure of former president Bill Clinton’s trial in 1999.

US CapitolThe Senate side of the US Capitol at sunrise in Washington DC (AP)

– Drawing the curtain

“At all times,” according to senate rules, a majority of senators present can vote to close the proceedings and debate in private. That would mean the TV cameras will be shut off and everyone who is not a member of the senate will be kicked out of the chamber until the senators choose to reopen it.

Senators did just that at various points during the Clinton trial. Mr McConnell then argued that members of the chamber listen to each other better in private.

Senators are sworn inSupreme court chief justice John Roberts swears in members of the senate for the impeachment trial (Senate Television/AP)

– The long haul

After the four days of opening arguments – with a maximum of 24 hours per side – senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate. Only then will there be votes on calling other witnesses.

Senate rules say the trial must proceed six days a week – all but Sunday – until it is resolved.

– Off the grid

Watch out for a coterie of Democratic senators who literally would rather be somewhere else – specifically Iowa and New Hampshire – ahead of their party’s first votes over the right to choose who will attempt to unseat Mr Trump in the November election.

Senators Elizabeth Warren of Massachusetts, Bernie Sanders of Vermont, Michael Bennet of Colorado and Amy Klobuchar of Minnesota may show signs of fatigue as they fly between Washington and these places – as well as coping with being off the internet for hours at a time.

Also look out for more video calls to supporters and ads designed to give them a measure of presence in the early nominating states.

Bernie SandersDemocratic presidential hopeful Bernie Sanders (APP)

– The prosecutors

Leading the case for the house is intelligence committee chairman Adam Schiff of California, and judiciary committee chairman Jerrold Nadler of New York. Five other Democrats round out the prosecution team, a group US house speaker Nancy Pelosi said she chose in part for their experience with the law.

Zoe Lofgren of California has worked on three impeachment inquiries, starting with the one that helped persuade former president Richard Nixon to resign.

Val Demings of Florida is not a lawyer, but she is a former police chief, and as member of both committees is deeply familiar with the case against Mr Trump. Hakeem Jeffries is a lawyer and chairman of the House Democratic Caucus, so he is close to Ms Pelosi’s ranks.

Ms Pelosi also chose two freshmen who helped flip the house from Republican control in 2018. Sylvia Garcia of Texas is a former judge. And Jason Crow of Colorado is a retired Army Ranger who was one of the seven new members with national security backgrounds to call for Mr Trump’s impeachment over his conduct with Ukraine.

– For the president

Donald Trump has cast some big personalities in his defence team.

White House counsel Pat Cipollone and personal lawyer Jay Sekulow are expected to lead the argument that Mr Trump committed no crimes, that abuse of power is not an impeachable offence and that the president is a victim of a political “witch hunt” by Democrats.

Bringing experience both in constitutional law and the politics of impeachment, he has added retired law professor Alan Dershowitz and Ken Starr, the independent counsel who investigated Mr Clinton. The team also will include Pam Bondi, the former Florida attorney general.

The team, less experienced in the senate than the house prosecutors as a whole, visited the senate chamber on Monday, in part to test the equipment they expect to use for audio-visual presentations.

Look out for signs of tension involving the president’s outside legal team and lawyers within the White House. On Sunday, Mr Dershowitz tried to distance himself from the president.

READ MORE: Donald Trump impeachment trial prosecutors unveiled by democrats

John RobertsChief justice of the United States, John Roberts (AP)

– In numbers

100: The total number of senators.

53: The Republican majority.

51: The number of senators who must agree on almost anything to make it happen during an impeachment trial.

Four: The number of Republican senators who must join the Democrats to get to the magical 51 level.

Two thirds: The proportion of senators required to convict and remove a president from office.

So, 67 members of the Senate would have to vote to convict, if every senator is voting.

Nancy PelosiSpeaker of the house Nancy Pelosi (AP)

– The gang

Both sides will be keeping tabs on the senate’s moderates for an emerging gang of three to four who could influence the outcome on such matters as whether to subpoena former national security adviser John Bolton. That vote will not be taken for days, if not weeks.

Republican senator Susan Collins of Maine has been meeting with a small number of her party colleagues who want to consider witness testimony and documents that were not part of the house impeachment investigation.

Watch Republican senators Mitt Romney of Utah, Lamar Alexander of Tennessee and Lisa Murkowski of Alaska for signs of whether this group can stick together and force the senate to consider additional material.