A Socialist government in France? It hasn't happened since the Fourth Republic of the 1950s -- so long ago, in fact, that even the shrewdest followers of French politics are by no means certain what to think of it.

And some Americans, of course, may think it doesn't even bear thinking about, such is the image of years past of a French Socialist Party: far to the left, linked inextricably to a Communist Party ally.

But you might as well be braced for the possibility of just that outcome from today's runoff election between President Valery Giscard d'Estaing and his Socialist Party challenger, Francois Mitterand. At last reckoning, Mitterand was a slight favorite in the polls, which is not to say that the volatile French electorate may not swing late to the center.

This, then, is not so much a prediction as an examination of a possibility of a historic alteration not only of the French political landscape, but of Franco-American relations as well.

It needs to be noted in passing that even a Giscard victory would not be merely a perpetuation of the status quo: a continuing stability of sorts in French policy and performance; considerable French support for the emerging Reagan approach to East-West relations and the security of, among other things, the Persian Gulf -- what one authority describes as "Giscard's infatuation with a muscular United States."

The "infatuation" might still be there. But most experts think the first electoral go-around on April 26 laid bare significant weaknesses in the president's centrist power base. With the party lineups badly splintered -- the Communists battered, the Socialists and the right-wing Gaullists stronger -- even a triumphant Giscard would have a far weaker hand to play at home and abroad.

What you might expect from Giscard's reelection, then, would be less (perhaps a lot less) of the same. But what you have to expect from a Mitterand victory would be something profoundly different.

How profoundly? Both American and French students of the French political scene are quick to caution against anticipation of apocalyptic change. The socialists, they argue, are a pale copy of the doctrinaire party of a decade ago, whose economics were very nearly indistinguishable from those of their Communist Party partners. The disastrous defeat of the French Communist Party in the April 26 voting has weakened its power and its influence on the Socialists.

But even in their domestic social and economic policies, the Socialists in power would create serious problems for the United States. There would be radical internal economic "reforms," nationalization of key industrial facilities, some hostility to foreign (including American) investments, some wrenching of French relations with the European Common Market. "The net effect would be disruptive to monetary and trade relations," says one authority.

But it is the field of foreign policy that the coming to power of a Socialist government in France would have its greatest effect on current Haig/Reagan impulses to polarize the world and almost all of its problems in stark East-West terms; to seek to rally an anti-Soviet campaign in the Third World; to suffer authoritarian regimes in the interest of warding off the totalitarian (i.e., communist) encroachment. i

This is not to say that the French Socialists would take a "soft" view of, say, Soviet occupation of Afghanistan or intervention in Poland. But a Mitterand government, curiously, would be likely to restore a Gaullist distance from the North Atlantic Treaty Organization, ending the discreet collaboration with which Giscard has tempered the French withdrawal engineered by Gen. de Gaulle.

It is in the so-called Third World Where a Socialist government in Paris would be most likely to collide head-on with Reagan administration purposes. With its close ties to the Socialist International, the French Socialist Party has deep sympathy for far-left revolutionary movements -- including, you can be certain, the one in El Salvador. There would be no sympathy for the Reagan approach, say, to South Africa.

Perhaps most important, a Socialist win in France would cloud the seeming success of Secretary Haig in rallying NATO support last week for a harder line with respect to detente, arms control and the related question of nuclear arms deployment in Europe. A Socialist government in France would, as one expert puts it, "reinforce all the tendencies in Europe -- and especially in West Germany -- working against Reagan's policy."

And not just in Europe: There would be little stomach on the part of a Mitterand government for even discreet cooperation with the United States in the Perisan Gulf.

Those who support the Reagan policies, accordingly, would find real cause for alarm. Those with some reservations about the developing American policy, on the other hand, might well welcome a Mitterand government as a useful influence for restraint.