It’s only happened three times before in American history: a president facing impeachment proceedings.

• A political historian and professor at the George Washington University, warns against looking too much to the past for guidance
• Trump’s case centers on questions of abuse of office and his Ukraine call
• The fractured media also makes this instance different

With Congress now in the midst of an impeachment inquiry into President Donald Trump, can those past instances provide clues about what to expect this time? 

Matt Dallek, a political historian and professor at the George Washington University, warns against looking too much to the past for guidance.

“You can’t look at these past three examples and say, ‘Ah ha this is what’s going to happen,’” he said. “First of all, it’s not the way history works. It’s not the way politics unfolds.”

For starters, he said, each case was different, with its own individual set of circumstances.

Andrew Johnson was in a power struggle with congressional Republicans after the Civil War. Richard Nixon resigned before facing likely impeachment and removal tied to Watergate. Bill Clinton allegedly lied about his affair with a White House intern.

Trump’s case meanwhile centers on questions of abuse of office and his Ukraine call.

“Trump is such a strange and unique creature, and unique president,” Dallek said. “And the environment we’re in is also so charged in its own way.”

Beyond the particulars of each case, Dallek said politics are more polarized and partisan now -- which could impact how the impeachment case proceeds.

With most lawmakers in lockstep with their party, for example, it is difficult to imagine 20 Republican senators breaking from the president and voting to remove him.

The fractured media also makes this instance different, Dallek said. A multitude of cable and online outlets -- several with specific partisan tilts -- enable political echo chambers that foster division.

“Had Nixon had Fox News or Rush Limbaugh and the conservative media sphere, maybe he could have been saved,” Dallek said.

Despite the uniqueness of this moment, Dallek said there are some comparisons to be made. For example, today’s Republicans appear to be using a playbook similar to the one the Democrats utilized during the Clinton impeachment: defending their party’s president by calling the investigation partisan.