Only in America could an untested, unpredictable politician leap to the highest, most powerful position in the land. In most other advanced countries, Donald Trump simply couldn't happen.
And even when a Trump-like candidate does win office elsewhere, few democratic countries give their leaders the kind of power enjoyed by US presidents — including the ability to launch nuclear weapons entirely at their discretion.
These loopholes in the American presidential system opened a path for Trump, but they've also opened a pit beneath his candidacy.
Increasingly, the same party insiders whom Trump circumvented during the primary are turning against him. And so are a range of conservative policy experts — those who understand the responsibility that comes with the Oval Office, and who fear that Trump doesn't.
Absent a quick and dramatic turnaround, the result could be a death spiral that ends with one of the biggest general election landslides in recent American history.
Trump’s sinking ship
For the last two weeks, Trump's campaign has been trending steadily downward. As of Wednesday morning, he trails Hillary Clinton by roughly eight percentage points in the polls, while betting markets peg his odds of victory at roughly one in four.
Panic is spreading through Republican ranks, as more and more insiders wonder whether it's time to break from the flagging candidate. District by district, downballot candidates are being forced to do some uncomfortable electoral math, asking themselves whether to decouple their electoral fortunes from the party standard-bearer. Sometimes the answer is no, sometimes yes, but the question in increasingly unavoidable: Has Donald Trump become so unpopular that it's better to stand alone than to stand with him?
Open defections remain fairly limited, but they do seem to be increasing. In recent weeks, Senator Susan Collins of Maine and a handful of sitting congressmen announced that they won't vote for Trump.
Meanwhile, conservative intellectuals and policy experts have turned against Trump in a more concerted way.
A group of 50 former national security official signed a letter this week saying they would not vote for Trump because he "lacks the character, values, and experience" to be president.
Trump found a loophole in American politics
In a different kind of political system — say, with a prime minister — none of this would be happening.
To become prime minister, you generally need to serve in parliament. And more than that, you also have to be anointed party leader by your insider peers. So there's little chance of an open struggle between insiders and their candidate — since the insiders handpick the candidate.
Even Italy's Silvio Berlusconi, the flamboyant media-mogul turned prime minister, came to power by winning his own seat in parliament and building a coalition of supportive parties.
But America's presidential system is different. You can become president without any support in Congress. You just have to win over voters.
Trump was an in-between case. Having swung Republican voters to his cause during the primaries, he earned the somewhat-grudging support of party insiders.
But with some Republican lawmakers now repudiating Trump, he looks increasingly like the kind of outsider candidate that American electoral politics makes possible. A politician in chief who is beholden to voters, and dismissive of party politics.
The US political system makes Trump more dangerous
What happens if an outsider candidate like Trump actually wins the presidency? What constraints exist for a president with little respect for Washington experience, someone for whom the daily compromises of political life are less familiar and less dear, someone used to running a business empire where decision-making is highly concentrated and where whims can be readily turned into reality?
Really, though, that's just the most glaring example.
Throughout the campaign, Trump's distance from the world of political norms and established party practices seem to have given him greater latitude to reach for unfamiliar or even unworkable solutions — such as forcing Mexico to pay for an immigrant-blocking wall or banning Muslims from traveling to the United States.
Even more ominous is Trump's occasionally casual attitude toward violence. During the primaries, a series of assaults at Trump rallies was met with blase responses from the campaign. And this week a peculiar joke about how to prevent Clinton from nominating gun control-friendly Supreme Court justices has erupted into a controversy about whether he was slyly implying that "Second Amendment people" might assassinate her (or the judges).
It's simply not clear where Trump sets a limit on acceptable political violence, or how he might channel this violence as president, in the name of his agenda.
Republicans are out of options
They can't drop him from the ticket. It's simply too late; the convention has already happened. And they can hardly count on him bowing out on his own accord.
They also can't do what a normal parliamentary party would do: pick a new leader. That's not how America's presidential system works. The Republicans didn't choose Trump; the voters did, and the party played along.
What remains is a version of what we've been seeing, where individual Republicans politicians and conservative intellecutals turn their backs on Trump, hoping that it scuttles his chances or sparks a late-day conversion to party dogma.
But it's a massive sacrifice whose emotional difficulty shouldn't be underestimated. Because in the end it doesn't just hurt Trump, it also helps Hillary Clinton.
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Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the United States. He can be reached at email@example.com. Follow him on Twitter @GlobeHorowitz.