So it’s time to talk about impeachment.

When he fired the FBI director James Comey, President Trump came perilously close to an impeachable offense, according to Harvard law professor Mark Tushnet. Why was it not impeachable? How does the process work, and what would it take to impeach him?

In 1787 Benjamin Franklin, during a Constitutional Convention debate about how to deal with an “obnoxious” tyrant, suggested that impeachment would be preferable to assassination, the method used to rid Romans of Julius Caesar and the English of Charles I. In 2010 Josh Chafetz of Cornell Law School, citing Franklin’s argument, suggested that we think of an impeachable offense as an “assassinable offense,” one for which the historical remedy was assassination.

Two American presidents—Andrew Johnson and Bill Clinton—were impeached, but the Senate-as-jury concluded that their offenses were not impeachable. Both were acquitted and remained in office. A third, Richard Nixon, resigned under the looming certainty of impeachment and conviction. Chafetz, who is not to be confused with Jason Chaffetz of Utah, concluded that though Johnson’s and Clinton’s crimes were not impeachable, Nixon’s indeed were.

“Nixon’s behavior was constitution-subversive—and therefore tyrannical,” Chafetz wrote in 2010. So Nixon’s offenses provide a standard by which to consider Trump’s case.

The framers gave considerable attention to the problem of how the states should deal with an “obnoxious” leader, specifying impeachment for “treason, bribery, or other high crimes or misdemeanors.”

Separately they defined the first offense, treason, as “levying War against (the United States), or in adhering to their Enemies, giving them Aid and Comfort.” Treason is not a likely charge in this case. Even though it might be said that Trump has given aid and comfort to Russia, treason is applied only in time of war and we are not at war with Russia. Only with North Korea does a formal state of war still exist. Al-Qaeda and the Islamic State might—or might not—be considered enemies for this purpose.

An article of impeachment for bribery is more plausible. That would require proof that the president acted to benefit Russia, or anyone else, in return for a gift or bribe. If, for example, it could be shown that Trump received a loan from a Russian bank and the loan was forgiven or interest-free, and Trump lifted the Obama-era sanctions against Russia, or aided Russian policy in some other way, that could be considered bribery.

The third category—high crimes and misdemeanors—is more general and refers to abuse of high office. It includes a wide variety of possible offenses by a president including perjury, misuse of authority or assets, intimidation, conduct unbecoming the office, refusal to obey a lawful order, obstruction of justice, among others.

So didn’t Trump’s firing of the FBI director who was overseeing the investigation of his campaign constitute an obstruction of justice? Not quite, but it’s close. The president has the authority to fire the FBI director for any reason, the investigation being theoretically unaffected by the firing. But if he were to ask Director Comey to end the investigation of his national security advisor Michael Flynn, appoint a clearly biased replacement for Comey, order an end to the entire inquiry, reduce funding or staff for it, or take any other concrete step to limit the investigation or interfere with it, that might be obstruction.

I say “might” because that judgment will be made first in the House Judiciary Committee, then in the full House, and finally in the Senate—all controlled by Republicans.

What about giving sensitive intelligence information to Russian diplomats in the oval office thereby compromising the sources of friendly allies? That’s sheer stupidity, and there’s no law against that. But it might be considered an abuse of authority or bribery if the president were receiving something of value from Russia.

So what was it about Nixon’s offenses that made them impeachable? Chafetz explains:

“The three articles of impeachment adopted by the House Judiciary Committee charged [Nixon] with obstruction of justice with regard to the Watergate break-in; using federal agencies (including the FBI and the IRS) to spy on, harass, and intimidate his political enemies; and defying congressional subpoenas in the course of the impeachment inquiry. The Watergate break-in was not some piece of petty burglary—it was a raid on the offices of the Democratic National Committee ‘for the purpose of securing political intelligence,’ in the words of the impeachment articles. By participating in the cover-up, Nixon made himself party to an attempt by his allies to use the levers of power to keep him in power. Likewise, the allegation that Nixon used federal agencies to go after his political enemies involved the use of power in an attempt to entrench power. Viewed this way, Nixon’s behavior has less in common with Clinton’s—despite the superficial similarity of both being charged with obstruction of justice—and more in common with the [Roman] First Triumvirate’s attempts to consolidate its power and Charles’s attempts to circumvent institutions meant to check royal power. Nixon, like Caesar and Charles, sought to aggrandize his office, arrogating to himself new powers and using them to entrench himself in office. His vision of the presidency was too big, not too small. Nixon’s behavior was constitution-subversive—and therefore tyrannical—in a way that neither Johnson’s nor Clinton’s was.”

The Nixon case, both straightforward and time-consuming, provides the modern example of the process.

In May 1973 draft resolutions calling for his impeachment were forwarded to the House Judiciary Committee, which hired 33 lawyers, including three women—one of whom was Hillary Rodham—and spent eight months investigating.

In July 1974, after three months of formal hearings, the committee voted out and sent to the House the three articles of impeachment. A simple majority vote in the House would have impeached Nixon, a stage comparable to indictment. A two-thirds vote in the Senate trial, overseen by the chief justice of the Supreme Court, would have convicted him, completing his removal from office.

Though both houses of Congress had Democratic majorities and Nixon was a Republican, it took more than a year to amass the necessary simple majority in the House and two-thirds in the Senate. On August 7, 1974, the Republican leaders of the House and Senate and the Republicans’ senior statesman Sen. Barry Goldwater went to the White House to tell Nixon that he was finished, his support in Congress evaporated, that the votes were there for the impeachment articles to pass both houses. Two days later he resigned.

Trump is blessed with stubbornly supportive majorities in both houses. How long that support will continue in the face of his childish and unstable behavior remains to be seen. The ultimate authority lies in the people who elected those Republicans. And they must lose their stony patience.