PARIS — Paris. -- Margaret Thatcher said in the early 1970s that 10 years are about as long as any democratic public will put up with a given leader. She failed to profit from her own observation and leave the prime ministership when she was at the peak of her accomplishment and reputation.
Instead she stayed on, and found herself humiliatingly ejected from 10 Downing Street last year by the vote of her own party's parliamentary members.
Francois Mitterrand was elected president of the French hTC Republic 10 years ago May 10. Is it the prudent time for him to go? Probably.
However his term of office extends for another four years, and in only two years, if not before -- and before seems rather likely -- he must lead his Socialist Party into a new parliamentary election it may well lose. The course ahead is likely to prove downhill.
This is too bad, because the French president is a remarkable and intelligent man who has completed that moral as well as political remaking of France which Charles DeGaulle began 51 years ago with his broadcast from London defying France's surrender to Hitler.
The collapse of democracy in France in 1940, as well as of national resistance, had been prepared for years. It was in part a consequence of the bloodletting of 1914-1918 -- which left every European country in shock -- but it was also the product of an old political intolerance with roots in the French Revolution itself. It is this which Mr. Mitterrand and Gen. DeGaulle together have ended.
The prewar French had failed to find a peacetime equivalent of that ''sacred union'' of which they were capable in wars. Instead, their political life was marked by intemperate and unforgiving struggles between social groups as well as political parties, few of them prepared to concede as legitimate the interests of rivals.
The left -- ostensibly revolutionary, anti-clerical, anti-bourgeois -- held power in modern times only twice before Mr. Mitterrand's victory in 1981. It governed in 1924-26, and again, in the Popular Front, in 1936-37. Both episodes ended in financial panic and the government's collapse.
When the Nazi invasion came in 1940, the traditional French right, monarchist, Catholic, anti-foreign, anti-Bolshevist (''better the Germans than the Reds''), socially reactionary, gave up the country to German domination with the argument that France had to be purged of its rot, and that Marshal Philippe Petain would lead France back to the standards of an idealized past.
The accomplishment of DeGaulle, in defying capitulation, was to lay the groundwork for a moral reconstruction of France. He created a new union of the conservatives and nationalists prepared to fight for France, with the socialists and Communists who would also do so, and who envisaged a postwar France of greater social justice. He reconciled the French to one another after 1945 by deliberately promoting a myth of generalized national resistance.
He and his associates established educational and industrial institutions that in the four decades which followed made the country into a modern democracy. His crowning institutional accomplishment was the constitution of the Fifth Republic, providing a strong presidency and a restricted parliament.
Francois Mitterrand famously described that constitution as ''a permanent coup d'etat.'' But as everyone knows, once Mr. Mitterrand was elected to that presidency, he employed its powers with relish and effect.
He became president by doing for France's left what DeGaulle had done for the right: He liberated it from its myths and ideologies. He united Socialist factions during the 1960s and 1970s and then brought the Communists into an electoral alliance that made possible the national electoral victory of the left in 1981.
By bringing the Communists into government, he destroyed them as a political force, depriving them of what always had been their reason for political existence, their farouche hostility to all forms of established power. Once they held power, their followers deserted them -- often to the country's other radical opposition force, the rightist National Front.
He reconciled the Socialists with market capitalism and the modern international economy --doing so despite himself: Mr. Mitterrand has never understood economics, but he understands the demands of power. He made peace between the left and the Catholics and ended an old and sterile quarrel over religious vs. lay schools.
The most important thing he did was to demonstrate that the left could govern, reform itself and deal effectively with foreign nations and international affairs. Indeed, he so succeeded that today the Socialist Party has come to seem the ''natural'' party of government, while the parties of the right have foundered into factionalism.