Paris — Traditionally, French politics is marked by two constants: a sharp ideological fault line dividing left and right, and a tendency to spend the summer at the beach forgetting politics.
But this vacation season, President Francois Mitterrand broke both rules. He replaced his prime minister and forced the departure of the Communists from his government, launching a concerted assault on the middle ground.
While it remains far from sure that these startling moves will succeed in turning around the Socialists' ailing fortunes in time for parliamentary elections in 1986, Mr. Mitterrand has at least succeeded in taking the offensive away from the opposition.
The new prime minister, Laurent Fabius, is leading the charge. In a stream of visits around the country and in introductory conversations with business and trade union leaders, Mr. Fabius has won generally favorable reception for his clear, straightforward talk.
His message is austerity. France must limit its government deficit and modernize its industry, Mr. Fabius says, even if that presupposes more large layoffs like last week's firing of nearly 6,000 Citroen workers, and the proposed firing of one-fifth of Creusot-Loire's 12,800 workers in an effort to save that heavy-engineering firm. To encourage businessmen, he has put an accent on less state interference and action to boost profits.
This tough pragmatism has confused the conservative opposition. Austerity with a pinch of liberalism, after all, is the basis of its economic programs.
''We cannot speak irresponsibly,'' declared neo-Gaullist assemblyman Philippe Seguin. ''In economic policy, we would have done much of what the Socialists are now doing.''
Mr. Seguin's statement sent shock waves through the conservative ranks. The opposition - already split among three potential leaders, Paris Mayor Jacques Chirac, former President Valery Giscard d'Estaing and former Prime Minister Raymond Barre - now also is arguing on its attitude toward the government.
Mitterrand also hurt the opposition with a deft tactical maneuver, proposing to hold a constitutional referendum on whether to permit further referendums on matters of civil freedoms. It added up to a referendum on a referendum - admittedly foolish and unimportant sounding - but potent because it made Mitterrand appear to be a defender of freedom while once again putting the opposition on the defensive.
After another embarrassing public debate, the conservative majority in the Senate finally voted to kill the referendum.
The entire affair offered the conservatives a no-win choice. Approving the plan would have risked giving the Socialists a sort of confidence vote. Opposing it risked appearing narrowly partisan.
While the opposition bickered, the government took every opportunity to hold the high ground. For example, when Prime Minister Fabius appeared in Toulon to mark the 40th anniversary of Allied landings in southern France, he declared, ''France is never so strong as when it is united.''
Old ideological disputes have been buried to prove this commitment to national unity. In July, the government withdrew its controversial plan to close the predominantly church-run private educational system. Last week Fabius seemed to settle the issue with new proposals that essentially leave private education alone. The disputed reform of the press has been quietly shelved.
Nevertheless, Fabius must do much more to gain his desired ''unity.'' His centrist talk on the economy may have confused the opposition, but it also translates into unpleasant realities.
A report released in August by the Organization for Economic Cooperation and Development warned that the French government must expect unemployment to rise for the next 18 months. It also warned that any premature reflation of the economy would ruin the significant progress made in narrowing the country's trade deficit (less than $70 million in July) and its inflation rate (down to around 7 percent), and that further tightening might be necessary.
In the past few weeks the government has been forced not only to permit the large layoffs at Citroen, but also to increase gas prices and telephone charges to keep the budget deficit down.
Not surprisingly, this outlook has worried the traditional left. The Communists pulled out of the government precisely because of austerity's unwelcome side effects. Now, even moderate union leader Edmond Maire of the Confederation Francaise Democratique du Travail has warned publicly, ''It is time to remember our Socialist orientation.''
To a certain extent, Fabius can ignore these criticisms. With unemployment so high, the unions seem unwilling to risk losing even more jobs by calling a strike. The Citroen layoffs, for example, have been accompanied by angry union statements - but no union action.
Still, Fabius will need union votes in 1986. And if he is to construct a new majority without the Communists, he will need voters on the right of the traditional left-right divide.
The most recent opinion polls show the electorate has not yet returned its verdict on the new government. Only 29 percent of the voters favored the Fabius appointment - but 56 percent preferred to ''wait and see'' before giving a verdict.
Only time will tell, then, how attractive Mitterrand's middle ground is.