This prediction has nothing to do with Portillo's gay past, at least not directly. On the few occasions that I have spent any time with him, Mr Portillo has always been charming and rather intriguing, attractive qualities in a politician. But, I repeat, I do not believe that Mr Portillo will get the job that he so obviously seeks.
There is too much excitement, verging on hysteria, around Mr Portillo - "the darling of the right" - and has been for several years. I am told by a close political ally of his that he has been both relieved and encouraged by the newspapers since his "gay" disclosures last week. But if I were him, I would be worried.
Yesterday's press was typical. There were the inevitable interviews and articles with men who had featured in Mr Portillo's past, combined with articles from right-wing columnists heralding him as their great saviour. It is all too intense, with a slight hint of danger.
It is true that the most successful politicians have an aura, but they tend to have acquired this once they have become leader of their parties, rather than several years before. Tony Blair evokes a Portilloesque excitement and has done since he won the leadership of the Labour Party in July 1994. But before then he did not rouse the passions greatly. He was able to reinvent himself from the Labour candidate who defended the party's 1983 election manifesto to the great moderniser, while away from the relentless gaze of the media.
Margaret Thatcher had an aura that even her political enemies could not deny. But in her case it took even longer to acquire. She started to dazzle only in 1981, after being in office for about two years. Likewise, Harold Wilson did not excite before he became leader of the Labour Party in 1963, and Ted Heath lacked that certain something until he became Prime Minister in 1970.
Given all this, Mr Portillo has been burdened by an aura for far too long.
The Times hailed him as the Tories' new "philosopher king" in 1993 after a vacuous speech, couched in epic, biblical language, about the need to cut public spending. When John Major resigned in July 1995, to stand against himself as party leader, it was Mr Portillo's supporters who leapt into premature action by installing those infamous telephone lines. When a Portillo celebration was called for, Alexandra Palace had to be the venue.
A Portillo fringe meeting at a Tory conference has, in recent years, been the political equivalent of a rock concert. The lavish, doting, packed crowds at such gatherings come close to those full houses addressed by Tony Benn in the early Eighties.
But Mr Benn is not Labour's equivalent of Portillo. Peter Mandelson provides a much closer parallel. Mr Blair's close ally has had the knack of provoking the same sort of media frenzy, with near-fatal consequences. For years, whatever Mr Mandelson said or did became front-page news. Shortly before his resignation late last year, a close ally told me that it was almost impossible for him to be taken seriously. Mr Mandelson was one of a select group who had filled the media void created by the death of Diana, Princess of Wales.He was closer to being a celebrity than the plain old Secretary of State for Trade and Industry. That was only one manifestation of the hysteria around him.
For years, the media and many in his party had credited him with near- magical powers to transform it. Those round him were touched with the same aura. Derek Draper worked for him? Give him a chat show! Charlie Whelan hated him? Give him a chat show as well! All those around him were sucked up in the hysteria.
But it helped to bring about his downfall. For Mr Mandelson was never as heroic or as monstrous as he was depicted. The reality was always more complex. Of course, Mr Mandelson half-enjoyed the myth and was partially responsible for its creation. But I got the impression that, in his view, the whole thing had got dangerously out of control, and there was nothing he could do about it.
The same applies to Mr Portillo. He has, at times, consciously contributed to his aura, but now there is nothing he can do to control it. I believed him when he told The Times that he had not wanted an ostentatious party at Alexandra Palace, and that it had been arranged by eager local party workers. But Mr Portillo's own personal view is strangely irrelevant. Indeed Portillo, the person, ceased to matter long ago. Portillo the Great Political Crusader, who has big parties, plots against leaders and has convictions that will transform the Tory Party, drowns out the real version.
Why is this dangerous for him? After all, there are many politicians - William Hague springs to mind - who would love an aura, and die for the plaudits Portillo gets in most right-wing newspapers and the devotion he attracts among his followers. Surely there could be no better springboard for a challenge after the election against a defeated, lacklustre leader?
I do not see it like that. Over the next few years, Portillo will never be out of the limelight. His every word will be weighed for its significance. Was his declaration of loyalty to Mr Hague really a subtle stab in the back? Were his comments on the euro a sign that he was wobbling? What does he really mean by a "tolerant" society? Show us the policies that accompany the inspiring rhetoric. And, unlike that of Mr Blair, Portillo's reinvention will be covered hour by hour, day by day, as it happens.
This will be an immense pressure for a politician to bear and, one way or another, Mr Portillo, who has erratic political judgement, will slip up. Either he will say something to upset his doting right-wing fans or he will, inadvertently, convey the strength of his ambition and make enough enemies in the party to block his leadership bid when it finally comes. Do not forget that the Portillo myth united enough voters in Enfield (where he was a hard-working MP) to produce the most spectacular example of tactical voting ever seen.
There are other reasons that call into question Mr Portillo's chances. In some ways these are much easier to pinpoint than the destructive frenzy that I have described. He will be on the losing side of a referendum on the single currency, because Mr Blair will hold one only when he is certain of victory. If the Tories lose heavily at the next election on a right- wing, Eurosceptic manifesto, is it so certain that they will choose another right-wing Eurosceptic as their new leader? On the other hand, if Mr Hague can narrow the gap his party may just give him another chance, as Labour did with Neil Kinnock in 1987.
But, more than all these dry factors, it is the uncontrollable hysteria that will finish Portillo off.