This week, US President Donald Trump repeated a threat he first made in 2016 when running for the presidency: that he might ignore the constraints on taking office imposed by the US Constitution, and in this case, seek a third presidential term. This should alarm anyone who cares about democracy worldwide.

Leaders who overstay their constitutional term almost always go on to wreak havoc on a wide range of democratic constitutional norms. In part, this is because they gain the power to appoint so many more public officials who are then toothless in upholding the Constitution for years to come. Even the mere fact of a president overstaying can also undermine the rule of law because it signals that those at the highest levels of government are unwilling to respect the law.

No matter who we are – and regardless of our politics, or where we live – we should therefore oppose the American president’s threatened path.

In 2016, the threat was about not respecting the results of the presidential election, in his contest against secretary of state Hillary Clinton. In 2020, Trump made a similar threat, suggesting he might not leave office, even though the results of the presidential election pointed to an apparent electoral victory by Joe Biden.

And, of course, many of his supporters sought to make good on that threat on January 6, 2021, with a violent attack on the US Capitol building, designed to sway the process of certifying Biden’s win. US prosecutors alleged Trump was instrumental in those attacks. I have argued the same thing in The New York Times – and called on the cabinet and Congress to remove him because of it.

Now Trump is trying a different tack. He is saying he is fit, healthy and “would love” to run for re-election in 2028. And his supporters are preparing the groundwork.

They are not just selling over-priced Trump 2028 caps. They are suggesting the law is on his side, not against him, in this bid – despite the language of the 22nd Amendment of the US Constitution providing that: “No person shall be elected to the office of the President more than twice”.

One path they point to is Trump being elected as vice president, then taking back the presidency. Another is becoming speaker of the House, and then president again, via the Presidential Succession Act.

Comparative experience suggests another possibility: that Trump could claim an implied exception to the 22nd Amendment, in cases of non-consecutive re-election. (The argument here would be that “elected twice” actually means “twice in a row”).

Similar forms of creative interpretation underpinned attempts by presidents to overstay their constitutional term in Bolivia, Burundi and Senegal, among other countries. In the US, Trump would also have on his side the fact that the Supreme Court often takes a less-than-literal approach to constitutional interpretation.

This argument, however, is both wrong and dangerous – and should be pre-empted, from now, by a loud chorus of constitutional experts and concerned citizens. As the Council of Europe’s Venice Commission has noted, presidential term limits serve a range of important functions: they give voters the chance to experience different forms of leadership, in ways that reduce the chances of charismatic leaders gaining dictatorial power through false claims that their leadership is necessary for national prosperity.

Term limits also prevent a president from gaining the power to appoint too many other independent office-holders, including judges and electoral and integrity agency heads who should be responsible for holding the president to account. This was the key reason the Constitutional Court of Colombia held that a president should never be able to gain a third term in office in 2010. It is also the most important rationale for the 22nd Amendment.

Some might say the danger becomes less when a president serves three terms but non-consecutively. This can sometimes be true – because it can reduce a president’s power to “stack” or undermine other independent institutions. But in Trump’s case, the problem is that he has already made a range of significant appointments to independent constitutional oversight bodies – most notably to the Supreme Court and lower federal courts, which themselves have no term limits or mandatory retirement age.

The current Supreme Court is composed of six justices appointed by Republican presidents, and three appointed by Trump (Neil Gorsuch, Amy Coney Barrett, Brett Kavanaugh). These same judges have upheld Trump’s claims to broad presidential immunity and to appear on the 2024 ballot in Colorado after a lower court in that state purported to ban him from running because of his actions on January 6, 2021. (Barrett was the only one of the three to suggest meaningful limits to this immunity principle.)

A third term, for Trump, is thus a true third opportunity to undermine commitments to democratic constitutionalism and the rule of law in America.

So far, Trump has not succeeded in gaining office unconstitutionally. And he is not a young man; rather, he is an old one, well known for floating ideas he does not always follow through on. Let’s hope this is another one. But sometimes, people get third-time lucky. And Trump’s luck, in this context, would truly spell bad luck for the rest of us.

It would give him even greater power to destroy American constitutional democracy, and its system of constitutional checks and balances, from within. And it would send a signal to would-be authoritarian leaders around the world that they are free to do the same.

For Australia, that could mean a nuclear ally without constitutional checks and balances – and a region with even fewer reliable neighbours. A third term for Trump is thus something we should all loudly reject – now and into the future.

Rosalind Dixon is Anthony Mason Professor and Scientia Professor of Law at UNSW Sydney. She is also co-director of UNSW’s new Resilient Democracy Lab.