On Wednesday, hours after acting Attorney General Rod Rosenstein appointed a special counsel to investigate Russian electoral interference, Fox News host Sean Hannity opened his show by describing a vast conspiracy to bring down the president.

There are five groups “aligning to stop President Trump,” he said: “the destroy Trump media… the Democrats, totally unhinged… perhaps the most dangerous, the deep state… the never-Trumpers… and number five, establishment Republicans.” In short, virtually every politician and civil servant in Washington.

Hannity, one of Trump’s most loyal defenders in the conservative media, sounded rattled. His monologue urging voters to “fight back” was an attempt to shore up Republican support in this moment of crisis for the young administration. Impeachment is a matter of politics, not law.

Democrats have fantasised about removing Trump from office ever since his election victory. A lawsuit brought by Citizens for Responsibility and Ethics in Washington argues that the president’s conflicts of interest put him in violation of the emoluments clause of the Constitution, which prohibits officeholders from accepting gifts from foreign entities. Others on the left have speculated whether the Twenty-Fifth Amendment, allowing for the replacement of a president deemed mentally unfit, might apply.

The stark reality is that in more than two hundred years, only one president has been impeached by his own party, Andrew Johnson in 1868, and only one, Richard Nixon, has resigned. Impeachment requires a simple majority in the House and the approval of two-thirds of the Senate. The definition of “treason, bribery or other high crimes and misdemeanours” is notoriously elastic.

In a recent NBC/Wall Street Journal poll, 78% of respondents called for an independent investigation into possible collusion between the Trump campaign and Russia. Rosenstein’s choice of special counsel, Robert Mueller, is the only man to lead the FBI for a full ten year term in the modern era, and is held in high esteem by both parties for his willingness to stand up to executive power.

Mueller’s remit is broad. His investigation will pick up where Comey’s left off, and cover “any matters that arose or may arise” - including possible obstruction of justice. Trump’s admission that he fired Comey because he was dissatisfied with the Russia investigation is prima facie evidence, as is Comey’s memo stating that Trump urged him to go easy on former National Security Adviser Michael Flynn, but intent to obstruct justice is hard to prove, particularly where presidents are concerned.

In the short term, Mueller’s appointment will slow things down and provide political cover for Republicans, lessening the pressure on congressional committees to conduct thorough oversight, but as Evan Osnos pointed out in the New Yorker, “investigations beget investigations”.

Kenneth Starr was appointed as a special prosecutor to scrutinise Bill and Hillary Clinton’s role in an Arkansas property deal, and ended up uncovering evidence of the president’s affair with Monica Lewinsky, leading to Clinton’s eventual attempted impeachment on charges of perjury and obstruction of justice.

The Russia investigation could yet turn out like the Iran-Contra affair - a huge scandal that hampered Ronald Reagan in his second term but ultimately only reached as far up the chain of command as Defence Secretary Caspar Weinberger. If Mueller’s investigation stops at Flynn and Trump’s campaign manager Paul Manafort, the president will declare himself vindicated.

The great Democratic hope is that it will go the way of Watergate, expanding and deepening until the president has no option but to resign. The analogy between these two investigations into electoral shenanigans, aided by an adversarial press, is appealing but flawed. Nixon weighed his words and actions like the lawyer he was, whereas Trump vents on Twitter, apparently heedless of the potential consequences.

The FBI concluded that the Watergate break-in was part of a sabotage campaign on Nixon’s behalf in October 1972, but the president hung on for almost two years by refusing to hand over incriminating White House tapes and firing special prosecutor Archibald Cox. He eventually resigned in August 1974, after leading Republicans Barry Goldwater, Hugh Scott and John Jacob Rhodes told him to go.

Trump’s Republican caucus has been notably restrained in its criticism so far. Senate Majority Leader Mitch McConnell has expressed a wish for “a little less drama” at the White House. “Can we have a crisis-free day?” wondered Senator Susan Collins. “That’s all I’m asking.” The following day, a transcript of Trump telling Russian officials that Comey was "a real nutjob" was leaked to the press.

Two upcoming by-elections, in Montana, which Trump won easily in November, and a deep red district of Georgia, will indicate how much of a drag the president is on his party. Nixon was damaged by the loss of two House seats in 1974, in elections Republican candidates were favoured to win. If Democrats can cause an upset in Montana or Georgia, it may cause Trump’s congressional backers to reassess.

Republicans hold a twenty-three seat majority in the House of Representatives. When a president’s approval rating is below 50% going into midterm elections, on average, his party loses thirty-six seats. Trump’s approval rating is currently 39%.

If Democrats retake the House in November 2018, they will run committees with the power to demand Trump’s tax returns. Impeachment charges will become a realistic possibility, although the Senate would almost certainly dismiss them, unless Trump’s standing is so disastrously reduced that the GOP tosses him overboard.

In an official statement, Trump welcomed Mueller’s appointment: “A thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity.” Maybe so, but he appears vulnerable. On Friday, it was revealed that a senior White House adviser, rumoured to be Trump's son-in-law Jared Kushner is a "person of interest" in the FBI investigation. Hannity’s call to man the barricades was a reminder of how few true friends the president has left.