LILLE, France (AP) _ The Socialist Party, preparing for presidential elections next spring, closed its bi-annual congress Sunday hampered by the lack of a candidate.

The three-day congress featured repeated attacks on the performance of Premier Jacques Chirac's conservative government. But the Socialists remained conspicuously short of new ideas to convince the electorate they could govern better than the right.

The speeches in this northern French city were less utopian in tone than in the years before 1981, when Francois Mitterrand won a seven-year term as president and the voters elected a Socialist-dominated National Assembly for five years.

The French rejected the Socialists in March 1986 and returned the conservatives to power in the Assembly, creating what is known as ''cohabitation '' between a Socialist president and a conservative premier.

Lacking a clear-cut decision from Mitterrand about his intentions for 1988, the congress in Lille was limited to obligatory calls for unity among the party's factions and a lambasting of the ruling center-right coalition.

Most party members are solidly pro-Mitterrand and most Socialist leaders want to see the president run again. Mitterrand says he is not inclined to run again, but he has not denied he will do so.

Waiting in the wings unable to start running are a number of potential candidates, including former Agriculture Minister Michel Rocard, by far the most popular candidate after Mitterrand.

Until recently, former Premier Laurent Fabius was also considered a possibility, but his star appears to have faded as Rocard's has risen.

Over the weekend, Mitterrand's first premier, Pierre Mauroy, implicitly gave his backing - and that of the pro-Mitterrand faction - to Rocard in the event the president does not seek re-election. That led the pro-Socialist daily Le Matin to crown Rocard the ''vice-candidate.''

The party's factions agreed they will back either Mitterrand or another well-placed candidate.

But until the choice is made, party officials cannot begin to put together an election platform, since they don't know what the candidate will accept. That means most policy decisions have been put off at least until a party convention at the end of this year.

In a closing speech to 15,000 people, party First Secretary Lionel Jospin emphasized that the situation is different from other presidential elections because the Socialists now have an incumbent president and five years of experience in running the country.

Responding to the argument that the right has only had a year in power, while the Socialists were rejected after five full years, Fabius responded, ''Time is precisely what is aggravating things.''

Recent polls have indicated that the public is re-evaluating the Socialists' performance between 1981 and 1986. After a year of rightist government, many now view the Socialist record as better than it seemed a year ago.

Jospin admitted that the Socialists fought a defensive campaign in 1986 and that their ideas were not always clear.

Jospin's outline of ''orientations'' for the 1988 campaign stressed traditional Socialist themes, such as improved job training, fiscal measures to promote investment, a global strategy for employment, reduction of the work week, solidarity with the poor and the fight against inequality.

Speaker after speaker emphasized that the party was firmly anchored on the left, rejecting the oft-expressed idea that to win the Socialists must ally themselves with the center of the political spectrum.

The center ''is nothing but a wing of the right,'' Jospin said.