French politicians both in and out of power have been using the public forum provided by the approach of next week's Williamsburg summit to shore up their own positions at home.

The latest example of domestic political maneuvering in the shadows of the summit was provided by a 75-minute-long meeting today between President Francois Mitterrand and his predecessor, Valery Giscard d'Estaing. It was the first time the two men had met since the Socialist election victory in May 1981.

One explanation for the meeting is that it formed part of Mitterrand's attempts to create a national consensus before doing battle with President Reagan and other western leaders at the weekend meeting on behalf of France's beleaguered economy. Another is that, while he and Giscard were ostensibly discussing international affairs, the subject uppermost in their minds was their own slippage in the public opinion polls in recent months.

There was a curious symmetry about the way in which each leader insisted that his only concern was the greater well-being of France while hinting delicately that his rival might be motivated by baser political instincts.

Speaking to reporters in the courtyard of the Elysee Palace after the meeting, Giscard said that Mitterrand had found it "useful to consult me" in view of "France's difficult and uncertain situation." He added that he found it "normal" to share his experience with the present incumbent of the Elysee despite their "deep disagreement" on how to govern the country.

At the rival briefing, government spokesman Max Gallo said the president was quite well aware that the opposition leaders had their "particular point of view" to defend but that nevertheless he felt it his duty to consult with them on the "great international issues" of the day.

It was left to the hardened political cynics to note that Mitterrand's public approval rating has slumped during the past four months from 46 percent to around 32 percent--the lowest level for more than 20 years--and that Giscard consistently trails other opposition leaders in the popularity stakes.

A clue to Mitterrand's possible motives in receiving Giscard was provided by the normally well-informed satirical newspaper, Le Canard Enchaine, this week. It quoted members of the president's entourage as saying that the move was a sleight of hand to boost Giscard's standing at the expense of his two principal rivals in the opposition, Jacques Chirac and Raymond Barre.

According to this argument, Mitterrand's long-term interest lies in playing opposition leaders off against each other. Socialist Party officials consider Giscard the least of their worries.

Mitterrand went out of his way to treat his predecessor with all the deference due to a former head of state. Breaking with his normal practice, he personally greeted and said goodbye to his guest at the door of the Elysee.

For his part, Giscard is also very well aware that his strongest card in a rather weak hand is his status as a former president. In order to keep pace with the more dynamic Chirac, the neo-Gaullist mayor of Paris, and the more respected Barre, his last prime minister, he has adopted a strategy of speaking out on major world issues.

The preoccupation with domestic politics also helps explain the divergent tones that have been struck by French spokesmen during the preparatory period for Williamsburg. Mitterrand has presented himself as a campaigner against American economic imperialism, which allegedly is deliberately maintaining high budget deficits, high interest rates and a high U.S. dollar.

The private message being put about by French officials, on the other hand, is that they are reasonably happy with a compromise that seems likely to emerge at Williamsburg. A senior French official noted with satisfaction today that the Reagan administration is doing everything possible to make the summit a success.

The French, as is often the case, seem to be playing a classic double game. They have a definite interest in bolstering the western alliance, which they consider to be under threat from a blustering Soviet Union and a weak-willed West Germany. They also have an interest in keeping the Americans in reserve as a handy scapegoat.

Earlier this year, Mitterrand approved a stringent austerity program. In doing so, he temporarily rejected an alternative strategy that was being urged upon him by some of his leftist advisers who wanted France to bail out of the European Monetary System and develop its own industry behind protectionist barriers.

Political analysts here believe that, by combining loud criticism of American monetary policies with quiet diplomacy to smooth over the differences at Williamsburg, the French president is keeping all his options open.