President Francois Mitterrand is likely to introduce a measure of proportional representation into the electoral system in order to increase his chances of completing his seven-year term, analysts here say.

Politicians and political analysts have based this conclusion on their study of results of local elections, widely seen in France as a dress rehearsal for crucial national legislative elections next year.

Today's second and final round of the local elections confirmed a sharp shift away from the left to right-wing political parties opposed to Socialist President Mitterrand. If repeated at a general election under the present winner-take-all system, the Socialists and Communists could be reduced to as few as 100 seats in the 474-seat National Assembly.

An exit poll conducted among voters during the first round showed that had the French had been voting for the National Assembly, 55.2 percent would have voted for right-wing candidates and 41.9 percent for the left.

Socialist Party strategists have said that they will be analyzing the results of the local elections closely before making a final decision on electoral changes. They have defended themselves against opposition charges of gerrymandering by pointing to a long tradition of French governments changing the electoral system to their own advantage.

Next year's legislative elections have assumed a particular importance as they are being viewed as a test of the stability of the political institutions of the French Fifth Republic ushered in by Gen. Charles de Gaulle in 1958. The remarkable stability of the system up until now has depended on the president's ability to control the National Assembly.

Political analysts believe that Mitterrand sees proportional representation, which was a plank in the Socialist election platform of May 1981, as a means of reducing the risk of a potentially destabilizing confrontation between a right-wing assembly and a left-wing president.

France's leading political weekly, L'Express, reported this week that former prime minister Pierre Mauroy had told Mitterrand that the introduction of proportional representation was "the only method of saving your presidency."

The effect of proportional representation would be to increase the number of seats in the National Assembly allocated to the fringe parties and reduce the number of deputies belonging to the mainstream right-wing parties. Mitterrand's advisers believe that this might enable the president to govern from the center during the remaining two years of his presidency until 1988.

Opposition leaders have said that proportional representation would favor the extreme right-wing National Front, which collected around eight percent of the vote in the local elections. Under the present system, minority parties such as the National Front run the risk of being squeezed out in the second round of voting.

An analysis of voting habits in the local elections showed that most National Front supporters were transferring their votes to other right-wing candidates in the second round. Voters for the mainstream opposition, however, were hesitant about backing candidates for the National Front, which has exploited the issues of immigration and security.

Computer projections showed that the left-wing parties had made a slight comeback in the districts involved in runoffs between the two rounds of the local elections. Analysts warned, however, that today's voting did not give an accurate national picture as it involved only about one-third of the electorate.

Exit polls showed that the high level of unemployment, which has reached 2.5 million, was the most important issue in the election, followed by the rising cost of living.