The French presidential campaign descended into mudslinging of an intensity rare in political life here in a last-minute bid by both sides in the close contest to gain the favor of French voters going to the polls Sunday.

The tactics seemed to be a measure of the extreme tightness of the race matching President Valery Giscard d'Estaing, the candidate of the center-right, against his Socialist challenger, Francois Mitterrand.

As the nation observed a legally imposed campaign moratorium today in the last 24 hours before the vote, the Socialist leader was generally given a slight edge. The head of the leading French polling group, JeanMarc Lech of ISOP, told French journalists that in seven polls his group has taken since the election's first round April 26, Mitterrand has been clearly ahead. But a commonly heard comment among French voters was, "I can't really believe we are about to do this."

If Mitterrand gains a victory despite the traditional reluctance of French voters to endorse the left in the second round of the country's two-round system, it will be a test of the Fifth Republic institutions tailored for the late Charles de Gaulle in 1958. Mitterrand's election would mark the start of a period of uncetainty over what ruling coalition he would try to form and new legislative elections that he has said he would call.

With Gaullist voetes -- 21 percent in the elimination round -- holding the key to the outcome, much of the mud was slung about resistance records from World War II. What candidates did during the war is still considered a touchstone for many Gaullist voters.

Prominent supporters of both candidates accused the adversary of being involved in collaboration with France's German occupiers and of being latecomers to the resistance. In both cases, the historical record was bent unflatteringly.

There was a major outcry this week after Gen. Alain de Boissieu, De Gaulle's son-in-law, said he would resign as chancellor of the Legion of Honor rather than give Mitterrand the symbols of the president's formal leadership of the legion, the most prestigious French national order. De Boissieu said he could not stand the thought of decorating a man who had so insulted his father-in-law and was a Johnny-come-lately to the resistance.

Four members of the legion's governing board resigned in protest, charging that De Boissieu brought the nonpartisan legion into politics. In addition, several of Mitterrand's adversaries pointed out that De Gaulle had always attributed honorable wartime contributions to Mitterrand.

Using a variant of the traditional campaign tactic of the incumbents to maintain themselves in power since De Gaulle founded the Fifth Republic, Giscard warned that the platform of the Socialist "collectivists" involves taking away pensions, doubling inheritance taxes and generally expropriating small property holders.

"A Metterrand victory would either mean Communist order or Socialist disorder," Giscard said.

Mitterrand solemnly listed what he said were 12 "lies" uttered in 20 minutes by Giscard during campaign oratory.Mitterrand added that the president had not dared to utter his "lies" during their lackluster two-hour debate Tuesday night.

"Shooting people in the back is his specialty," Mitterrand said. "it worked once with De Gaulle in 1969. But with me he is going to miss."

That was seen as an appeal to Gaullists who blame Giscard's public opposition in 1969 for De Gaulle's defeat in a referendum that led the wartime resistance leader to resign.

Giscard shot back that "only the truth hurts." He publicly read out some of the more radical proposals in the Socialist program. Mitterrand's loss of his calm disqualified him for the presidency, Giscard said, asking, "Where is 'the tranquil power?'"

This was a reference to the reassuring campaign slogan Mitterand used in an attempt to give voters the message that his election would not involve social upheaval.

Until the final outbursts, it had been a campaign that elicited frequent public complaints of dullness as Mitterrand deliberately maintained a low profile with few public meetings, in an apparent effort not only to reassure, but also to avoid having to give up his deliberate ambiguity over his real preferences between Communist and Gaullist voters, both of whose support he needs to win.

Gaullist leader Jacques Chirac, who got 18 percent of the first-round vote, came out relatively strongly for Giscard after earlier practically telling his followers that they need not follow his example in voting for the incumbent.

Privately, Mitterrand strategists were quick in the final week to stress that their man has the best chance of keeping the Communist Party down to the 15 percent result its candidate got in the first round. This was the party's lowest showing since 1936. But such high-ranking Socialists took care to avoid saying anything publicly that might turn Communist voters away from them.

The weakened Communist leadership, confronted with wholesale defection by voters who rejected the party's almost open sabotage of Mitterrand for more than three years, endorsed him without conditions.

The Soviet party leadersip did not hesitate to indicate its preference for Giscard. In a runoff election eve comparison of the foreign policies of the two French candidates, the Soviet party newspaper Pravda complained that Mitterrand's attachment to detente is watered down by his insistence on "firmness" and that he also insists that the Soviet Union remove its SS20 missiles aimed at Western Europe in any negotiation about the East-West nuclear balance in Europe.

That kind of commentary on Mitterrand is a clear reminder that during the 11 times he was a minister before the Fifth Republic, he always proved himselp to be a moderate -- so much that many leftists say they are planning to vote for him holding their noses.

At 64, Mitterrand has been around far longer than the 55-year-old Giscard in French public life. But Giscard suffers not only from having to assume responsibility for France's economic ills. His strong control of radio, television and a large portion of the press has resulted in an overexposure that seems to be backfiring just as his image has become increasingly tarnished by such affairs as his acceptance of diamonds from the deposed Central African emperor, Jean-Bedel Bokassa.

Mitterrand said yesterday that when Giscard challenged him to give the exchange rate of the West German mark during their debate, while they were still trying to seem gentlemanly toward each other, that he had been tempted to challenge the president to give the current price of diamonds. In his final televised campaign appeal last night, Mitterrand recalled what is undoubtedly his most telling single argument -- that Giscard's reelection to a second seven-year term will mean 14 years of the same president.