THE QUESTIONS

What is an impeachable offense?

The Constitution specifies two crimes — treason and bribery — for which the president can be impeached, as well as a third, broader category of “other high Crimes and Misdemeanors.” It is generally understood that to the drafters of the Constitution, the word “high” referred not to the severity of a given crime but to the abuse of high office.

Otherwise, though, what falls under the category of impeachable offenses is a matter that floats free from the rest of the American legal code. The slippery nature of the concept spurred Gerald Ford’s famous declaration in 1970 that “an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” That’s true, but which offenses should Congress consider impeachable? The debate over how to answer that question revolves around a couple of hypotheticals:

A crime that’s not an impeachable offense?

“I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose voters,” Mr. Trump famously said at a campaign rally during the 2016 presidential race. As it happens, an imagined scenario in which the president commits murder was raised as far back as 1789. Michael Stern, former senior counsel to the House of Representatives, writes that “there is no serious question that our murdering president could be impeached for his crime.”

But “not all crimes by federal officials have been seen as impeachable,” write Laurence Tribe, a professor of constitutional law at Harvard, and Joshua Matz, an adjunct professor at Georgetown Law, in their book “To End a Presidency.” So where exactly does the line fall? During Bill Clinton’s impeachment trial, Mr. Tribe argued that his conduct did not rise to the level of an impeachable offense because it related to his private life.

But Michael Stokes Paulsen, a professor of law at the University of St. Thomas, disagrees, claiming the distinction is contrived: “Perjury and obstruction of justice are serious felonies (even if somehow cast as ‘private-capacity’ wrongs),” he writes. “They are not quite like gunning down a man, but they are serious wrongs against the public and the administration of justice.”

Ultimately, the line between an offense that’s impeachable and one that’s merely illegal remains just as obscure as it was during Clinton’s trial. In the words of the Congressional Research Service’s Constitution Annotated:

There appeared to be broad consensus in the Senate that some private crimes not involving an abuse of power (e.g., murder for personal reasons) are so outrageous as to constitute grounds for removal, but there was no consensus on where the threshold for outrageousness lies.

An impeachable offense that’s not a crime?

In 1974, the House Judiciary Committee’s second article of impeachment against Richard Nixon alleged an abuse of power in his Watergate cover-up, a political judgment with a legal foundation. (Nixon resigned before his impeachment could be formalized.) In the case of Mr. Trump’s Ukraine call, some argue that his conduct amounted to an abuse of power that likewise violated both the Constitution and criminal law. Leah Litman, an assistant professor at the University of Michigan Law School, wrote for NBC News:

Trump sought to use the extensive powers of his office to get a foreign government to open an investigation that would be damaging to a political rival. … Using the office of the president for personal political benefit comports with both the standard understandings of bribery and the broader category of high crimes and misdemeanors.

Many caution that without an underlying criminal offense, like bribery, impeachment risks becoming a partisan political weapon.

The argument against this sort of ad hoc impeachment has its roots in Supreme Court Justice Benjamin Curtis’s defense of President Andrew Johnson during his impeachment trial in 1868. As Nikolas Bowie, an assistant professor at Harvard Law School, writes in the Harvard Law Review, Curtis believed that retroactively criminalizing a president’s behavior — inflammatory, racist campaign speeches — would violate a fundamental principle of common law: “There must be some law,” Curtis argued, “otherwise there is no crime.”

Mr. Bowie argues that the decision to impeach Mr. Trump without any statutory justification would set a dangerous precedent that “would apply not just to someone as unpopular as President Trump but also to future Presidents whose policies happen to misalign with a congressional majority.”