Tuesday marked the beginning of the Senate impeachment trial that will determine whether US President Donald Trump is removed from office. Since the House of Representatives – one half of the country’s legislative branch – impeached the president on 18 December, those interested in the forthcoming trial have looked to history to get a sense of what might be coming.

Only three US presidents have faced potential removal from office by way of impeachment; Andrew Johnson in 1868, Bill Clinton in 1998 and now Mr Trump. In 1974, Richard Nixon was facing possible impeachment but chose to resign before the House could vote on the matter.

Due to the small field of examples, it’s difficult to lean on precedent to glean insight into how the current impeachment trial will play out. Vice President Mike Pence penned an editorial in the Wall Street Journal last week comparing Mr Trump’s impeachment to that of Mr Johnson, the 17th president of the US. In his editorial, Mr Pence characterised the impeachments of both Mr Johnson and Mr Trump as “a partisan impeachment” over “a difference in policy” and that the supporters of the president’s removal have “cheapened the impeachment process.”

Whether Mr Pence’s appraisal of the current impeachment trial is fair or accurate is up for debate. That said...

There are some similarities between Mr Trump and Mr Johnson

Mr Johnson ascended to the presidency in 1865 following the assassination of US President Abraham Lincoln. Ultimately, he was impeached because he violated an antiquated law called the Tenure of Office Act. While that was the “on-paper” rationale behind the impeachment, it was well understood at that time that there were other factors driving the push for his removal, much as it is understood that withholding money from Ukraine isn’t why Mr Trump is facing trial in the Senate.

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

1. The “smoking gun” used to justify their impeachments isn’t the only reason their opponents want them removed.

Mr Johnson’s post-US Civil War policies empowered racist Southern governments to install Confederate leaders and pass laws that limited the rights of recently-freed former slaves. When Republicans – who were abolitionists – fought to fight the laws using legislation, President Johnson frequently vetoed them.

After a number of high-profile firings of congressional appointees, his Republican opponents drafted an impeachment resolution against him on the grounds that he violated the Tenure of Office Act, a law which has since been ruled as unconstitutional that forbade presidents from removing certain Congressionally-appointed individuals without seeking approval from Congress first. It is largely believed the law was instituted as a way to keep checks on Mr Johnson’s power.

Similarly, while Mr Trump’s impeachment is ostensibly a result of him abusing power in an attempt to solicit Ukraine’s help in the upcoming US presidential election, it’s clear that Democrats view the president as fundamentally unfit for office.

2. Both Mr Trump and Mr Johnson are painted as victims of witch hunts by their supporters

On Tuesday, Mr Trump referred to the impeachment trial as a “hoax” and a “witch hunt.” By his own words, Mr Trump believes he is being impeached for “making a perfect phone call.” Mr Trump’s supporters overwhelmingly see the impeachment as a bad faith attempt by Congressional Democrats to remove the president.

Mr Johnson was portrayed as the victim of an out-of-control partisan Congressional faction – dubbed the “Radical Republicans” – for more than a century before his efforts to undermine the rights of freed black Americans turned the tide of historical sympathy against him. Mr Johnson was saved from removal by the vote of Kansas Senator Edmond Ross, whose story is the centerpiece of Pence’s editorial and the focus of a chapter in US President John F Kennedy’s book “Profiles in Courage.”

3. Both embodied the kind of person they say they’re working against

Mr Johnson was a Congressman from Tennessee during the US Civil War and referred to Secessionists as “traitors” and was a proponent of harsh punishments for those he deemed treasonous. Despite his outspoken disdain for the Confederates, his positions after the war fell almost entirely in-line with their racist ideology.

Mr Trump has ridden a wave of resentment towards those he and his supporters dub “elites” directly into the oval office. Though Trump’s messaging to his base is that he is a beacon of hope for the common man, Trump’s worth has been estimated by both Forbes and Bloomberg at over $2 billion, his social life entails rubbing shoulders with the wealthy and he owns a number of buildings with his name prominently displayed on them.

4. Both trials generated massive media interest

The public was so enthralled with the impeachment trial of Mr Johnson that 1,000 tickets to the event were distributed daily, and police had to be mobilized to keep crowds from storming the Senate chambers. While the majority of these tickets went to Congressmen and their spouses or friends, most of the general public had to rely on newspaper reporting at the time.

While it’s unlikely that the trial of Mr Trump will generate ticket sales, it will almost certainly be the dominant story across all US media for at least the next few weeks. Senate Majority Leader Mitch McConnell will attempt to set rules for the trial on Tuesday that will limit the time senators have to hear testimony and view evidence in an attempt to move the trial along as quickly as possible. The longer the trial drags out, the longer it’s going to be a staple of the daily news cycle.

5. Neither one will be removed

Thanks to a vote by Mr Ross against his party’s interests, Mr Johnson was spared removal from office. After leaving office, he was re-elected to the Senate in Tennessee before his death five months later.