But he had a tough time under steady grilling by Philippe Seguin, the treaty's principal opponent. Given the President's deep unpopularity, it remains to be seen how last night's encounter will change the balance of support for the treaty, which is poised on a knife-edge.
In a two-and-a-half hour programme devoted to the subject, the 75-year-old Socialist was dignified, defiant and passionate in defence of the treaty, of which he was a key architect. His critics may have underestimated his ability to command an audience. He dominated the screen.
But Mr Mitterrand clearly did not foresee the force of Mr Seguin's critique, delivered in a clear, even, but respectful tone in the Gaullist politician's lugubrious manner. Mr Seguin had the advantage of time over the President, who had been arguing the toss for two hours before the two locked horns. It showed.
The President put up a strong showing in the first section of the programme. A rejection of the treaty on European union on 20 September 'will be without doubt a serious reverse for France and for Europe,' he said, adding that France must not miss 'the rendezvous of history'.
But Mr Seguin was firm in his opposition, saying that France was suffering a 'moral crisis', and that citzens saw 'the referendum as a chance to express themselves'. He added that the French 'resent the rise of technocracy'.
The President defended his decision to hold a referendum, saying how important it was 'to explain and simplify, without misrepresenting the text'.
Pressed as to whether he would resign if the referendum came down in favour of a 'no', Mr Mitterrand would only protest that he did not wish to drag domestic politics into the debate.
The much-heralded confrontation, involving the President, Mr Seguin, the cream of France's political journalists and a panel of the French public, was conducted in the splendid surroundings of the amphitheatre of the Sorbonne. It was a remarkable showpiece event, which at times had the air of a very high-class game show.
The President found himself pinned down by angry questioners from the public panel, but remained calm, if blunt. It was evident that he was under pressure - over the plight of French farmers, over the unemployed, over the Germans.
The electronic presence of the German Chancellor, who spoke over a video link, sparked strong protests. Mr Seguin complained that this was intervention in the internal politics of France. Helmut Kohl argued strongly for ratification, saying: 'There are moments of destiny for Europe, and this is one. The advice of a friend is: know how to seize the opportunity.'
The Prime Minister, John Major, refused an invitation to participate in last night's debate.
The involvement of the German spectre in the campaign has been a source of growing concern in France. But last night, Mr Mitterrand sought to redirect this against the 'Japanese offensive'.
(Photograph omitted)Reuse content