The impeachment process is a delicate blend of constitutional law and political calculation, with even the definition of the phrase “high Crimes and Misdemeanors,” which refers to allegations of misconduct by officials, remaining extraordinarily flexible. 

That phrase—part of Article II of the Constitution, which provides for the removal of the president, vice president and other civil officers for treason, bribery or other high offenses—was meant to be malleable from the start.

The framers of the Constitution “were sufficiently practical to know that no charter of government could possibly anticipate every future contingency, and they therefore left considerable room for play in the joints,” former Chief Justice William Rehnquist wrote in his book “Grand Inquests,” about the impeachments of President Andrew Johnson and Justice Samuel Chase. (Both men were acquitted.)

The concept of impeachment was taken from English law, where the definition of what constituted a removable offense was developed ad hoc, on a case-by-case basis. But the power was viewed at its core as a legislative check to protect the republic from executive abuses, said University of Missouri law professor Frank Bowman, author of “High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump. ”

“Certainly among the key ideas was that abuses of executive power would be impeachable,” he said.

President Trump’s interactions with Ukraine could open him up to three main types of abuse-of-power allegations, Mr. Bowman said: That he abused his authority as commander in chief, as well as his powers in the U.S. diplomacy and law enforcement arenas, by seeking to work with a foreign state against a domestic political adversary.

If the House of Representatives charges a president or other government official by approving articles of impeachment, a trial is held in the Senate, where a two-thirds vote is required for conviction.

It’s well accepted that not all crimes are impeachable. On the flip side, some alleged abuses of power have been deemed impeachable, even if they aren’t crimes.