MOST FRENCH VOTERS, and France's allies, must have read the election returns with the strongest sense of relief. Francois Mitterrand's election raised a possibility of deadlock between a Socialist president, committed to a leftist program, and a recalcitrant National Assembly. No doubt the prospect of stalemate, evoking recollections of the worst moments of French politics, contributed to the left's terrific victory in Sunday's first-round vote for the next assembly. It promises a legislature fully in harmony with the new president. The French believe, on the basis of much experience, that political division and weakness is never safe.

This ringing reaffirmation of France's turn to the left will inevitably create strains elsewhere in Europe. For the past seven years, most of Europe's crucial decisions were based on the close relations between the strong and assured governments of a center-right French president, Valery Giscard d'Estaing, and a center-left German chancellor, Helmut Schmidt. The emergence of a strongly Socialist France puts a new and highly inconvenient pressure on Mr. Schmidt in Germany. He has suffered a series of reverses recently in domestic politics, and the scale of the Socialist gains in France will now increase his difficulties in controlling the left wing of his own party. As the ambitions of the German left rise, they awaken apprehensions among Mr. Schmidt's indispensable allies on the other side, the small liberal party that provides his parliamentary majority. There's currently a wave of speculation in Germany that neither the chancellor nor his government will complete the four-year parliamentary term to which they were reelected last fall.

A party that comes to power in a landslide election, after nearly a generation in the opposition, has to deal with extraordinarily high expectations of its supporters. But Mr. Mitterrand and the new National Assembly are going to have to work under the close constraints represented by the European Community. France is now poised to pursue socialism in an economy that is highly integrated with the rest of western Europe's.

Last weekend's preliminary victory seems very likely to be confirmed next Sunday in the final round of voting. It would leave no doubt of Mr. Mitterrand's strength at home, and his ability to get the legislation that he wants. But the extent to which France can depart from the Western European consensus is less clear. Since Mr. Mitterrand has now overwhelmed his domestic opposition, the test of his statecraft may well be his ability to work with the other governments of Europe.