This is the text of a piece I’ve just written for BBC History Extra

A few questioned its necessity, but for most of the delegates to the constitutional convention in Philadelphia in 1787 giving Congress the right to impeach the President was an obvious move. It was not to be used lightly. It was an emergency break that might restrain a decline to authoritarianism. Outside the convention proto-democratic radicals warned there was great danger in investing in one man so many kingly privileges. What if a would-be dictator bribed his way into office? What if he colluded with foreign powers, or abused his power: would it be sufficient simply to wait until the next election to remove him? Impeachment was a possible solution.

It was a practice deeply rooted in English history and legal precedent, as was the phrase the Founders included in the Constitution as the criteria that would justify it: “treason, bribery or other High Crimes and Misdemeanors.” The Founders did not discuss in any detail what this phrase meant, but that was because they thought it was pretty obvious—it meant crimes against the state, not personal crimes. A president shouldn’t be impeached for dodging taxes, swindling people as a private businessman, or any other criminal offences that had no direct bearing on the exercise of his presidential powers. Such matters could be dealt with once the man left office (one respect in which the president retained kingly powers was in his apparent immunity from prosecution). Impeachment was not about punishment—it was simply about preserving the republic by removing from office someone who constituted a danger to liberty.

No one ever imagined impeachment would be anything other than a national trauma. “The prosecution of them,” Alexander Hamilton predicted in Federalist no. 65 (one of his essays promoting the new Constitution), “will seldom fail to agitate the passions of the whole community, and to divide it into parties, more or less friendly or inimical, to the accused. In many cases, it will connect itself with the pre-existing factions, and will inlist all their animosities, partialities, influence and interest on one side, or on the other; and in such cases there will always be the greatest danger, that the decision will be regulated more by the comparative strength of parties than by the real demonstrations of innocence or guilt.”

This was precisely why the constitution makes impeachment a tricky business. First, the House of Representatives needs to pass articles of impeachment—a list of charges—by a majority vote. At that point the President has been impeached but he has not yet been removed from office: his case then goes to the Senate, which convenes itself as a court and hears evidence on both sides. Only if the Senate then votes to convict by a two-thirds majority does the president then have to leave office.

Of 57 previous presidential terms, only one has ended in the president leaving office early without dying. That one example, of course, was Richard Nixon, who resigned less than two years into his second term, before his near-certain impeachment. On July 29, 1974, 28 out of 38 members of the House Judiciary Committee voted to impeach him on abuse of power charges, including some members of the President’s own party, suggesting that a large majority of the overall House would have followed suit. On August 5, the so-called “smoking gun” tape was released, proving that Nixon had sought to cover up his re-election campaign’s responsibility for the break-in at Democratic Campaign Headquarters in the Watergate complex during the 1972 election. In the light of this, his chances of avoiding conviction by the Senate faded. On August 9, he resigned. So although Nixon was not impeached, he was removed from office by the threat of impeachment. Alexander Hamilton might have thought this showed that the emergency break of impeachment was in good working order.

But ironically the only two presidents who were impeached by the House of Representatives—Bill Clinton in 1998 and Andrew Johnson in 1868—probably should not have been, at least not by the standards envisaged by the Founders. In neither case did the Senate convict, although in 1868 it came within one vote of doing so. President John Tyler also faced serious impeachment proceedings in 1842, though the matter was never brought to a full vote before mid-term elections that tipped the balance in his favour. Whig congressmen were furious with Tyler for using the presidential veto to strike down a new tariff bill. They accused him of abuse of power and former president John Quincy Adams was among those who recommended impeachment. (The Whigs were especially cross because Tyler had been elected in 1840 on the Whig ticket as the running mate of William Henry Harrison, who died less than two months after taking office: he was supposed to be one of their own!)

Gerald Ford, when he was the House minority leader during a controversy over the impeachment of a Federal judge, famously said, “an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” It’s a seductively cynical take. Can Congress pull the “emergency break” even when there isn’t a true emergency?

The impeachment of Andrew Johnson in the aftermath of the Civil War seems to prove Ford’s point. Johnson was a demagogic bigot, ill-equipped in any way for the moment in which he assumed office after the assassination of Abraham Lincoln. He was a Democrat from Tennessee who had been named Lincoln’s running mate in the 1864 election in an effort to show that Lincoln’s party could transcend the sectional divide. Coming to office, just as the Civil War was ending, he was soon at logger-heads with Congress because of his all-too-obvious sympathy for defeated white Southerners as he stymied every attempt by Congress to secure the rights of freed slaves or punish the leaders of the Confederacy. Even so, the basis on which he was impeached was ludicrous. Congress set a trap for Johnson that he angrily blundered in to. Congress had passed a “Tenure of Office Act”—later ruled to be unconstitutional—which stated that the President could not fire a cabinet member without congressional authorisation. Johnson duly walked into the trap by firing Secretary of War Edwin B. Stanton. Johnson could easily have been convicted by the Senate and, had he been, impeachment would have decisively tipped the constitutional balance towards the legislature, creating a quasi-parliamentary system in which the executive was effectively dependent on the approval of Congress. It is one of the big, and under-appreciated, might-have-beens of US history.

Bill Clinton probably did commit a crime when he lied under oath to a Special Prosecutor about his sexual relationship with a White House intern. But it was not one that met Hamilton’s standard of a crime against the state or a threat to individual liberty. Yet the Republican House majority impeached him anyway and most Republicans in the Senate voted for conviction. Only the two-thirds rule kept Clinton in office.

Conversely, in other cases, a President has had enough support in Congress to smother any moves towards impeachment hearings even when there was evidence that there might be something to investigate. There were mutterings about impeaching James Buchanan (1857-1861) and Ulysses S. Grant (1869-1877) for corruption, Harry Truman for abuse of power after he fired General Douglas MacArthur, and Ronald Reagan for his alleged role in the Iran-Contra affair – but in none of these cases was impeachment ever politically viable.

History therefore suggests that a pre-requisite for any serious moves towards impeachment is not the committal of an impeachable offense but partisan opposition from Congress. Even so, the severity of the alleged offences makes a differences – there are numerous examples of presidents who have faced a hostile Congress but who have never been anywhere close to impeachment, though the wilder fringes of the opposition demanded it (George W. Bush after 2006 and Obama after 2010 being recent examples).

The alleged offences and the clarity of the evidence matters—and it undoubtedly helps if the President’s crimes can be boiled down to a simple idea by the media (“he lied under oath” or “he covered up a crime”). Congress has sometimes taken seriously the views of constitutional lawyers and try to distinguish between what is truly an “impeachable offence” and what is not. For example, in his first term as president Richard Nixon seriously underpaid his taxes, which some at the time believed to be a criminal offense, but he wasn’t impeached for that, and nor should he have been.

What does all this tell us about the prospects for an impeachment of Donald Trump?

The importance of partisanship—more intense now than at any time since perhaps the 1860s—suggests that so long as the Republicans control the House there is zero chance of impeachment. But if the Democrats win a majority in November (and they are currently favourites to do so) that clears the way to possible impeachment proceedings.

The case that his actions are “impeachable” is vociferously made by pundits (including conservatives like Jennifer Rubin as well as liberals like Robert Reich), scholars like Alan Lichtman, journals like The Nation, Democrats (though so far not the party’s leadership, who fear it will backfire politically) and even a tiny number of dissident Republicans. The list of possible grounds for impeachment is a long one, including: abuse of power for firing FBI Director James Comey, undermining the freedom of the press by calling them the “enemy of the people” and for breaching the first amendment’s prohibition on the establishment of religion with his so-called “Muslim ban”. The requirement that he be subject to re-election was “not a sufficient security” thought James Madison. The president might “lose his capacity after his appointment. He might pervert his administration into a scheme of peculation or oppression. He might betray his trust to foreign powers.” And President Trump has been accused of all these things—of not being mentally stable, of abusing his office to enrich himself and his family, of conspiring with Russian secret services in the crime of computer hacking to aid his election, as well as of disclosing classified information to the Russian ambassador.

Yet even if Trump were impeached by a Democratic-controlled House next year his chances of being convicted and removed from office by the Senate would seem to be so low as to be virtually non-existent. Whatever happens in the mid-term elections, it is mathematically impossible for Democrats to gain a two-thirds majority in the Senate. 15-20 Republican Senators would be need to support the President’s removal from office, more than would have been necessary to convict Nixon had he not resigned first. Whatever they may think of Trump privately, Republican lawmakers have proved time and again that they will not take him on publically.

Only if Trump’s popularity with Republican voters plummeted might that change, but why would that happen? All the evidence suggests that Trump’s supporters have no illusions about his character but want him to be their President anyway. Nixon, by contrast, had very low approval ratings by the time of his resignation, but that was in a less partisan age in which there was no equivalent of Fox News to provide the president’s supporters with an alternative narrative. Barring death, Trump seems likely to remain in office until at least the next election so long as he maintains the support of core Republican voters.

And if Democrats know that impeachment would not lead to Trump’s removal, why would they try it in the first place?

The founders always knew that no constitution they devised could, in and of itself, preserve liberty and republican government. “Is there no virtue among us?” asked Madison. “If there be not, we are in a wretched situation. No theoretical checks—no form of government can render us secure.”

Virtue may seem a rather old-fashioned term in the age of Twitter, but if that’s the place we’re in now, impeachment certainly won’t help.