Nicolas Sarkozy, the hyperactive half-Hungarian who five years ago set his sights on the French presidency and has been plotting his way there ever since, passes a major milestone today as he becomes the centre-right's official candidate for April's elections at a triumphant party congress in Paris.

Two weeks ahead of his 52nd birthday - and after an internal party ballot in which he was the only contender - the interior minister and head of the Union for a Popular Movement (UMP) will be acclaimed by 50,000 jubilant supporters, transported from across the country in coaches and specially commissioned TGV fast trains.

After delivering a keynote address, the newly designated candidate will leave the pomp and fanfare of the Parc des Expositions and on Monday launch straight into the fight of his life: a three-month slog of speeches, press conferences and constant travel whose final destination is the Elysee palace.

All seems set fair for "Sarko", the ultra-ambitious right-winger who dreamed of politics as an adolescent, was mayor of the wealthy Paris suburb of Neuilly at the tender age of 28, and has not stopped since.

Polls show that he remains neck-and-neck with the Socialist candidate Segolene Royal, despite her advantage of non-stop media exposure since being nominated at the end of November. With his vastly greater experience, solid party backing and a clear manifesto, he has every right to fancy his chances in the rough-and-tumble of the coming campaign.

But one nagging doubt remains, which can be summed up in the name of Sarko's one-time benefactor and idol: Jacques Chirac. Treachery has a long tradition in French politics, and today treachery is once again in the air. Many years ago it was Sarkozy who allegedly betrayed his master. Now the fear is that the tables could be turned.

Chirac and Sarkozy have a complex relationship that goes back to the late 1970s, when the younger man attached himself to Chirac's rising star. In the early 1990s he was an essential member of Chirac's team, and rumours even spoke of a liaison with his daughter Claude. "And to think he used to see us in our dressing gowns," Chirac's wife Bernadette said later.

But in 1995 came the great divide, when Sarkozy backed Chirac's rival Edouard Balladur in the presidential race. Chirac, who went on to win the election, refused to speak to Sarkozy for several years. After 2002, when Chirac was re-elected, Sarkozy made no secret of his ambition to replace him at the country's helm - and the president has done everything in his power to thwart him.

What makes the rift more poignant is the obvious similarity between the two men. Sarkozy is often described as Chirac's "rebel son", and the single-minded purpose with which he has pursued his ambition reminds seasoned observers of the president's younger days.

Recently, as Sarkozy has backed away from calls for root-and-branch economic reform, castigated "yob bosses" and spoken out for state intervention, it seemed a re-run of Chirac's own famous policy tergiversations. Even the choice of the Parc des Expositions for today's congress has memories. It was there in December 1976 that Chirac launched the Rally for the Republic (RPR), the party that became his ticket to the Elysee.

Almost exactly 30 years later, the question is whether the 74-year-old president is willing to vacate it. Again and again he has had the chance of endorsing the young pretender, and every time he has refused. During his New Year greetings to the press on Thursday, Chirac failed to mention Sarkozy's name even once - instead letting it be known that he is himself giving "due thought" to running for a third term.

Meanwhile, Chirac's faithful lieutenant Prime Minister Dominique de Villepin is also chipping away at the UMP leader, refusing to vote for him in the party primary and giving interviews to newspapers about his plans for the next five years - as if it was not obvious to the entire country he will be out of office in May, whatever the outcome of the vote.

How far is Chirac willing to go to stop the man who would replace him? Could he even secretly favour a Socialist victory to wreck Sarkozy's career? Could he split the centre-right vote by running again and let far-right leader Jean-Marie Le Pen slip through once again into the second round, this time for a run-off against Royal? Today Sarkozy will justly be savouring his party triumph, but the questions will continue to nag.