By any metric, Donald Trump is in trouble.

A poll out from The Post and ABC on Friday shows that 60 per cent of voters disapprove of the job he's doing as president, a new low. But that's just one poll; the polling average at statistician Nate Silver's FiveThirtyEight shows Mr Trump with a mere 53.4 per cent disapproval rating, which is better than its 56.8 per cent peak last December.

But a presidency is not in good shape when the best spin on the new poll is "It's an outlier! Only 53 per cent of the country thinks the president is terrible." The poll is especially ugly for Republicans with midterms looming in two months.

FiveThirtyEight's forecast for the mid-terms puts the likelihood of Democrats taking the House at more than 70 per cent. Their chances of taking the Senate are lower, but Republicans are hardly a lock despite a very favourable map for them. And if Democrats manage to eke out a majority in both houses of Congress, here is the poll's really bad news for Mr Trump: Half the country wants him impeached.

To put that in perspective: In January 1974, well into the Watergate scandal, Richard Nixon's poll numbers on impeachment were better than President Trump's are now. Earlier, less disastrous polls for Mr Trump still showed him veering dangerously close to what we might call "the Nixon ceiling."

Most worrying for Mr Trump is that three-quarters of Democrats say they want Congress to impeach him. If Democrats gain control, they will be under immense pressure from their base to deliver.

That doesn't mean they'll do it. It takes a two-thirds super-majority in the Senate to actually remove a president from office. The best that Democrats can possibly manage in 2018 is a narrow majority; they would need more than a handful of Republican senators to support removal. The leaders of a Democrat-controlled House might well decide they'd rather not force their Senate brethren to take a hard and futile vote.

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But as Republicans found in the 1990s, these things have a way of taking on unexpected momentum. A former Republican congressional staffer who was close to that process tells me that the day after the bruising 1998 mid-terms, Newt Gingrich - who would shortly step down as House speaker - said, "impeachment is over, that's one thing the election clearly meant." Five weeks later, with Mr Gingrich out of the way, House Republicans impeached Bill Clinton. Then he was acquitted by the Republican-controlled Senate.

"The activist base of the party was committed to the idea, and that made it impossible for the elected officials to change course," the former staffer says, "even though they knew impeachment wasn't what the broader public wanted."

It's all too easy to imagine a similar scenario for Democrats intent on impeaching Mr Trump as they come up short looking for Republicans to help them make it across the finish line. But it's not entirely impossible to picture a few Republicans going along. If Democrats do manage to start impeachment hearings, it would be because - unlike Republicans in 1998 – they would be coming off a huge mid-term win. Public support for impeaching Mr Trump, even taking into account his more favourable polls, would be higher than it ever was for impeaching Mr Clinton.

Mr Trump is in a very unusual situation for an American president. Members of his die-hard base are loyal, but at his peak they were barely a plurality of the party. The rest of his support is purely expedient, interested in getting judges appointed and keeping Democrats out of power. Republicans in Congress are loyal, for now, but only because they're afraid of his voters.

But by the time Mr Trump faced a Senate trial, that would mean the political calculus had shifted radically. He would have cost them the Congress; there would be no hope of more judges; the 2020 election would seem already lost. And he'd have no reservoir of goodwill in the party, for at every turn he has made a point of attacking and humiliating any Republican he deemed insufficiently obsequious. Just how long will the Coalition of the Unwilling stand by a president who was never really their man?

But even if Republicans hold the party line, what Mr Trump faces in this scenario is bad enough: a public trial that he can't avoid by firing the investigators, nor distract from with more Twitter blasts. One senses that public humiliation, especially at the hands of an establishment that has always looked down on him, is the thing that Mr Trump fears most. Though far from certain, that humiliation is growing more likely.

Yes, the president is clearly in trouble. But does Mr Trump, hunkered down with deferential staffers and screens blaring Fox News, realise it? Or might he learn it only when Congress calls him to account – and he finds no one standing behind him?