Mr Seguin was courteous to the point of correctness when he debated the President on television last week. But he left Mr Mitterrand in no doubt that he does have enemies: those who believe that the Maastricht treaty, negotiated by him and the other 11 European heads of government last year, is detrimental to France. Mr Seguin is the main figure in the 'no' campaign, which threatens to bring down the President and the treaty when France holds its referendum on 20 September.
According to opinion polls, he helped to bring the level of opposition above the 50 per cent mark. But the latest survey last week indicated opposition support is falling back again. Now it is Mr Seguin who has a fight on his hands.
Mr Seguin, a 59-year-old deputy from the Vosges, is, superficially at least, another of the insurgent politicians who have stuck pins in the balloon of established authority over the past year. Like the leaders of regional parties in Italy who deflated the ruling Christian Democrats, like Pat Buchanan in the United States, he emerged to kick out the rascals. He has some of the props of a rebel: he is a chain-smoker, a point on which every profile picks (unfiltered Gitanes, naturally). He appears dishevelled, not the smooth type that characterises the highest levels of French politics. He talks about the Rolling Stones.
Yet he is in most respects a quintessentially establishment figure, with more than a decade in politics, including a spell as social affairs minister under Mr Mitterrand. He was educated at ENA, the elite school for French technocrats. He spent eight years in the civil service as an adviser, including a spell in the Elysee Palace. Yet now it is the technocrats and the Establishment against whom he rails in his attacks on Maastricht.
'I am not denouncing bureaucracy, it exists everywhere. Technocracy is a system where political power is wielded by technicians. It is completely different,' Mr Seguin said last week.
He was born and brought up in Tunisia before independence. His father died fighting for the Free French during the war and the young Philippe received his posthumous Croix de Guerre. He is a historian who has written several books, including a work on Napoleonic history. All of this gives him a deep affinity for France, its institutions and traditions.
As a man to lead a grassroots campaign, Mr Seguin has certain advantages. He is built like a pantechnicon, literally a heavyweight. When he trailed around France during the summer addressing rallies, he dominated each meeting with his physical presence and his quiet, confident delivery.
He has a hangdog expression and reminds some people of the French comedian Fernandel. His lugubrious manner, which is deliberate and measured, stood him in good stead when he faced Mr Mitterrand.
Mr Seguin is capitalising on a wave of anger that has built up in France over the past decade, years when long-term unemployment surged, farmers' incomes collapsed, and one man held power: Mr Mitterrand.
Populist movements like the 'no' campaign have arisen before against governments in France, a country that is deeply ambiguous about its ruling classes. Yet this is not purely a reaction by the lower-middle classes against the failures of the republic. Although it includes the two anti-regime parties, the National Front and the Communists, it also includes many solid citizens who would not dream of these extremes.
Opinion polls show that the Maastricht opponents tend to be older, and lower down the social scale. But there are plenty of counter-examples. Opposition to Maastricht cuts across class and age barriers. Butchers, bakers, candlestick makers, but also assistant analysts, doctors and accountants. 'We had always doubted this treaty,' a young lawyer said last week, 'but I think we would probably have voted 'yes' before the campaign began.'