At the outset of the Donald Trump administration, it seemed his most likely successor was his vice president, Mike Pence. However, it quickly became apparent that his UN ambassador Nikki Haley, who resigned last week, was at least as plausible.
She has used the UN post with consummate skill to promote herself as a national leader on the American right. Indeed, Mr Trump passed her over for Secretary of State in part because she had become too prominent and popular.
Ms Haley and Mr Pence are both former governors, and hence considered well-qualified for the presidency.
However, Ms Haley has distinguished herself repeatedly from Mr Trump, including implicitly criticising him and robustly pushing back against his implied criticisms of her.
Mr Pence, by contrast, has basked in Mr Trump's shadow. He is notorious for sycophantically praising Mr Trump and gazing at him with the puppy-eyed adoration Nancy Reagan reserved for her husband Ronald.
It is not clear when Ms Haley decided to resign, but there was virtually no advance warning from a White House that usually leaks like a broken bucket. But, whatever the proximate cause was, the obvious underlying reality is that Ms Haley is positioning herself for a presidential bid.
If Mr Trump continues to enjoy relative good fortune and virtually unchallenged support among Republicans, she will have to wait until 2024, when she will be just 52 years old. But, by leaving now, she is reserving the option of a 2020 bid, should one or more of the numerous potential crises on the horizon befall Mr Trump.
Leaving now is essential to preserving her viability as an alternative to Mr Trump, should he become embattled and weakened.
She would pose as a unifier in a post-Trump Republican Party, able to appeal simultaneously to the “America first” constituency because of her loyal service to his administration; to hawkish neoconservatives whose internationalist and engaged foreign policies she has supported; and evangelical Christians otherwise aligned with Mr Pence whom she has courted her entire career, beginning with a religious conversion to Methodist Christianity.
Ms Haley’s appeal will be considerable. Not only will she potentially be able to bring together the Trumpian, neoconservative and evangelical constituencies, she’s a potentially crucial symbol of diversity for a Republican Party now cripplingly identified mainly with white men.
She is also a relatively young woman of colour, an Indian-American of Sikh origin, and hence an important symbolic corrective to the Republican Party’s stronger-than-ever identification with older white males in a diverse society.
Americans generally look for a change after four or eight years, whether or not they’re switching parties, so Republicans probably need a striking contrast to Mr Trump to have a fighting chance after he goes.
And Americans may not relish following a white-nationalist President Trump with a Christian-nationalist President Pence, thus switching from ethnic to religious intolerance. Mr Pence’s Christian fundamentalism is very different to, and may be much less widely appealing than Mr Trump’s white ethnic chauvinism.
Ms Haley’s record on the Middle East is mixed but disturbing. Many Gulf audiences applauded her tough stance against Iran. In particular, she made the vital case that Iran is supplying the Houthis with the missiles being fired at Saudi cities.
Unfortunately, one way she used the UN post to further her political ambitions was by consistently and mercilessly bashing Palestinians. That pandered, at no political cost, to hawkish, neoconservative, evangelical and Islamophobic audiences.
Ms Haley scandalously blocked the appointment of former Palestinian Prime Minister Salam Fayyad as the UN special envoy to Libya, simply because of his Palestinian identity.
She strongly backed all of Mr Trump's endless, vicious anti-Palestinian actions, including recognising Jerusalem as the capital of Israel and moving the US Embassy there, and slashing US funding for Palestinian refugees
But the Palestinian view that Ms Haley was "the worst ever" American UN ambassador may not last long if the national security adviser John Bolton plays a key role in choosing her successor.
Mr Bolton will try to ensure that, unlike Ms Haley, Washington’s next UN ambassador is a relatively junior figure aligned with him. Indeed, if he can, Mr Bolton will even deprive her successor of full cabinet-member rank, which would both ensure his primacy and further denigrate the UN’s role.
But all of that would only underline how effectively Ms Haley has used her UN post to transform herself into a major national and international figure, and become extremely popular with the American right.
Even many “Never Trump” conservatives are bemoaning her departure and lauding her as one of the last of the “grown-ups” in Mr Trump's administration.
She did help keep the US internationally engaged, but often in an extremely destructive manner. And she never challenged Mr Trump’s white nationalism or, as Defence Secretary Jim Mattis has, fought for better policies within her own remit.
To the contrary, Mr Trump and Ms Haley generally seem to have brought out the worst in each other.
Nonetheless, she is now, more than ever, the clear heir apparent, and even a potential rival, to Mr Trump at the very top of the Republican party.