PARIS, SEPT. 14 -- With a few sharp blows discreetly below the belt, France's main political contenders have leaped eagerly into the eight-month campaign season for presidential elections.

Some analysts have suggested the elections are particularly important because they could encourage a shift away from ideological left and right toward a flexible center more dependent on personality and style. In addition, these analysts argue, the outcome will help determine how political power is divided over the long term between president and prime minister under France's hybrid 1958 constitution.

"These elections will decide not only the next seven years, but a long time to come," said a well-placed Socialist Party official.

President Francois Mitterrand, a Socialist elected in May 1981, has maintained a consistent first place in opinion polls on the upcoming election. But Mitterrand, 70, has created a deliberate mystery about whether he will run for another seven-year term.

In the meantime, he has sought to project an Olympian aloofness from day-to-day politics to preserve the nearly regal presidential image that has earned him the widest support of his 40 years in politics.

Mitterrand has said he will announce his intentions only at the last moment, remaining president and not candidate as long as possible. This has crimped the action of another Socialist pretender, former agriculture minister Michel Rocard, whose campaign would likely be eclipsed by Mitterrand's candidacy.

Prime Minister Jacques Chirac, whose conservative coalition won a majority in parliament in March 1986, has failed to transform his victory over the Socialists into the irreversible ascension to the presidency that his supporters had hoped for then. But according to followers in the neo-Gaullist Rally for the Republic, Chirac's decisions have set the stage for economic improvements likely to draw more voters as the day nears.

Chirac's recent attempts to enhance his image with young voters, including a photo session with the rock singer Madonna, have produced charges of clumsiness from Mitterrand's supporters. One joyfully pointed out that the 54-year-old Chirac, in a recent pose listening to a Walkman and wearing tennis shoes and a sweatshirt, forgot to change his sheer formal socks.

Former prime minister Raymond Barre, Chirac's chief rival on the right, has ridden his reputation as a brilliant economist to a high ranking in opinion polls and a widespread assessment that he would be the strongest candidate against Mitterrand. Barre has been cited as the right's best chance to make inroads among Mitterrand's supporters on the moderate left.

Jean-Marie Le Pen has taken his ultraconservative National Front on a nonstop campaign since announcing his candidacy last spring. Le Pen has been credited with only about 10 percent of the vote. But he has assumed an importance beyond the number of his supporters by forcing other political leaders to deal with widespread resentment of North African immigrants.

Although the conservative coalition in parliament apparently reflects a rightist electoral majority in the country, Chirac and Barre have entered the race with the distinct disadvantage of their own rivalry, a Chirac activist pointed out.

Mitterrand, if he runs, would be all but certain to win the first round of the election and become the leftist candidate without a bruising battle. Chirac and Barre face a tough battle in the first round to decide who will be the conservative candidate in the second.

"Between Chirac and Barre, whichever one is candidate in the second round, the other one will be behind him," predicted Jean-Pierre Bechter, a member of Parliament from Chirac's party.

The season-opening accusations flew between supporters of Chirac and Mitterrand. Chirac received the first blow when the Iranian speaker of parliament, Ali Akbar Hashemi Rafsanjani, revived charges that some of the prime minister's political supporters had interfered in the preceding government's efforts to win release of French hostages. When several Socialist leaders suggested an investigation was in order, Chirac responded that he, too, had some charges to raise.

Chirac specifically mentioned the Greenpeace affair, in which the previous Socialist government was caught blowing up an antinuclear group's ship in New Zealand, and the Vincennes affair, in which special police operatives working under Mitterrand reportedly planted evidence against Irish Republican Army suspects arrested here.

A hooded figure claiming to be a renegade French spy appeared on television last week to denounce the Socialist government's handling of the espionage agency involved in the Greenpeace affair. Then Justice Minister Albin Chalandon announced over the weekend that Christian Prouteau, an antiterrorism adviser to Mitterrand, should be indicted in connection with the questionable police work at Vincennes.

Socialist ex-prime minister Pierre Mauroy, a friend of Mitterrand, accused Chirac of "low political operations" and added: "We are awaiting the rest of Chirac's autumn soap opera, while regretting that the prime minister's dignity and functions are affected by it."

Mitterrand then demanded a Defense Ministry investigation of soldiers who detained several journalists in New Caledonia. The reporters, who were released after interrogation, were to cover a referendum ordered by Chirac and opposed by Mitterrand.