Regal image propels French President's rising star. The rising popularity of Francois Mitterrand is the political story of the summer in France. But that does not mean he is a shoo-in for next year's presidential election. The President is keeping everyone guessing about whether he will be a candidate. And if he does run, some analysts predict his popularity will not hold once he enters the political fray.

What a difference a few years can make. In l984 Socialist President Francois Mitterrand was perhaps France's most disliked president, barely polling support from one-quarter of the population.

Voters blamed Mr. Mitterrand for throwing the economy into a tailspin with his Socialist policies. When parliamentary elections were held in spring, 1986, the electorate turned instead to the Conservatives' dynamic, free-market policies, and President Mitterrand, as head of state, had to appoint Jacques Chirac Prime Minister. This was the first time any French head of state was forced to appoint a member of the opposition as the head of government.

Today, Mitterrand is one of the most popular French leaders since World War II, regularly winning approval ratings of more than 55 percent.

He has emerged as a reassuring ``grandfather of the nation.'' If the presidential election were held today - they are due in the Spring of next year - he probably would poll 54 percent of the vote compared with 46 percent for Mr. Chirac.

But, Raymond Barre, a conservative former Prime Minister who refused to join the ``cohabitation'' government, also has benefitted from Chirac's decline. In the latest polls Barre has pulled almost even with Mitterrand.

Mitterrand's resurgence could mean big change for the French political landscape. If he does win a second seven-year term (he entered office May 1981) he probably will call for new National Assembly elections. Currently, his Socialist Party holds just 38 percent of the seats.

No one here expects a Mitterrand government to return to the aggressive Socialist policies of the 1980's. But it no doubt would back away from Chirac's strong commitment to opening the French economy as well as his drive to denationalize and deregulate the French economy.

In part, Mitterrand has benefitted from the turmoil that has wracked Chirac's right-wing coalition government. Every time Chirac has a problem, Mitterrand's popularity gets another boost, says Claude Imbert, the editorial director of Le Point magazine.

Chirac has faced major setbacks over the last year, including strikes by students and transit workers, the defeat of a proposal to alter France's immigration laws, and a resurgence of terrorism.

His right-wing coalition also is fragile. Francois Leotard, Chirac's Minister of Culture, provoked a major storm this spring when he announced he would not support Chirac as the conservative presidential candidate next year.

``The further down Chirac falls, the further up Mitterrand climbs,'' says Olivier Todd, the former editor of the newsweekly L'Express.

He stays far removed from the day-to-day fray of politics, appearing to be above the dirty business of running the government. It is an image particularly appealing to the French, who have traditionally liked dominant, patriarchal leaders.

Charles de Gaulle accentuated the trend when he rewrote the constitution in 1958, creating a near imperial Presidency. Ironically, Mitterrand spent much of his political life battling de Gaulle. Yet, he has come to personify the larger-than-life President better than any French politician since the General.

In short, Mitterrand has turned himself into a sort of modern-day, democratically elected king.

``Mitterrand even has the air of a traditional monarch. If he put on a powdered wig, he'd look just like Louis the 14th,'' says Philippe Moreau-Defarges, an analyst at the French Institute of International Relations.

That's a far cry from Mitterrand's early reputation. In the 1950's he was known by critics as a politician of shifting loyalties, a Machiavellian opportunist who was none too trustworthy.

He started as a young minister in several right-wing governments, coming late to Socialism. He took over the dying Socialist Party in 1971, when it barely polled 5 percent of the electorate. Using his political skills and personal influence, he has brought about its resurgence as the premier party of the left.

To foster his current image, Mitterrand has positioned himself as the conscience of France, a defender of the national heritage.

The President only speaks out publicly on questions of defense or international affairs, or when he can cite constitutional issues. More than once, he has managed to come off as the defender of the constitution against what are perceived to be the vagaries of Chirac's government.

Mitterrand jealously guards the public perogatives of the Presidency. At the Venice summit of the western industrial powers and Japan held earlier this year, he headed the French delegation, not Chirac. Mitterrand meets alone with the other world leaders as head-of-state, and it usually is he who speaks for the French position.

While he avoids controversial domestic problems, Mitterrand hasn't lost his political touch. The President manages to appear non-partisan, yet his comments often seem to exacerbate simmering disagreements within the right.

For example, Mitterrand recently condemned the racism and divisiveness of the ultra-right wing National Front. It was a position few could argue with.

Nevertheless, Mitterrand's speech came as Chirac and his coalition were debating how far the ``respectable'' conservative parties should go to woo the National Front's extremist supporters. While the conservatives came off as political mercenaries calculating how many votes they might pick up by wooing a group most French do not support, Mitterrand had the look of a statesman.

Perhaps the best example of the President's political gamesmanship is his simple refusal to announce whether he will stand for re-election. The tactic frustrates the conservatives. Unable to start campaigning against him, there is little they can do to drive down his popularity. Chirac recently complained that Mitterrand actually has begun to run, making political speeches while hiding behind the mantle of head-of-state.

It's an argument that falls on deaf ears at the Elys'ee Palace, the home and office of French Presidents.

``It's not surprising that the right-wing is beginning to snipe at the President as they see him rise in the polls,'' says a Mitterrand spokesman. ``It doesn't bother us but I can sure understand why it bothers them.''

Some question whether Mitterrand's current popularity will hold once he declares his candidacy.

``Right now he's above the fray, beyond the realm of the other politicians. But the day he becomes a candidate, he will be judged by all his errors and actions of the past,'' says Alain Lamassoure, a conservative member of the National Assembly.

For that reason, Mitterrand probably will delay the decision for as long as possible. He has nothing to gain, and everything to lose by entering the race too early.

``He doesn't want to end up like Giscard,'' says political analyst Pierre Hassner. Former President Valery Giscard d'Estaing held a huge lead six months before the election in 1981, only to lose to Mitterrand.

In the end, most observers here think Mitterrand will run only if he believes he will win. Now 70, it may be his last chance to leave a mark on the French political scene.