Probably the most closely watched race in France's parliamentary elections this Sunday and next is the battle in a lower middle-class Paris district between two candidates who probably could be called the two most powerful women of the French Left.

They are incumbent National Assemblywomen Gisele Moreau, 40, the only woman in the seven-member Secretariat of the French Communist Party, and National Solidarity Minister Nicole Questiaux, 50, the highest ranking woman in the Socialist Cabinet and a key associate of President Francois Mitterrand.

"Gisele Moreau is the only credible candidate of the Left," said an impassioned member of the Communist Politburo speaking last night in the windup of her campaign to hold onto the seat she first won three elections ago. "Nicole Questiaux should have gone to do battle some place where the Right must be eliminated," he added in the cheers of about 150 persons in a covered school playground on the village-like Place Jeanne d'Arc in the 13th Arrondissement of Paris.

Moreau was easily reelected in 1978 with 53.3 percent of the vote in a runoff against a Gaulist. But in late April, Communist Party leader Georges Marchais got only 16 percent of the vote in this Communist fief (compared to Mitterrand's 30 percent), and the Socialist leader went on to sweep it in the runoff two weeks later with 60 percent, his best vote in all of Paris. Marchais' showing was 10 percentage points lower than Moreau's first-round result of 26 percent in 1978.

So, Questiaux's chance of knocking off a highly visible Communist leader is rated very high and has come to symbolize the triumphant Socialists' determination to reduce the Communists to an auxiliary force in the next National Assembly. Two other Secretariat members are threatened by Socialist challengers.

Mitterrand ran more stronglyt than Marchais in 44 of the 86 districts represented by Communists in the 491-member National Assembly. Top Communists concede that they expect to lose at least half their seats to Socialists.

The same unpublished poll that accurately predicted that Mitterand would get 52 percent to defeat incumbent president Valery Giscard d'Estaing on May 10 is now understood to be showing the combined forces of the left getting 55 percent in Sunday's first round, probably enough to give the Socialists a near-majority on their own.

The poll credits the Socialists with 37 percent on the first round, almost as much as the combined vote of the Giscardists and Gaullists, and 15.5 percent for the Communists, the same as Marchais' disastrously low result on April 26.

Candidates who get 50 percent or more of the vote Sunday will be elected outright. In districts where no candidate receives at least 50 percent, those receiving at least 12.5 percent will be in the final runoff the following Sunday.

Gaullist leader Jacques Chirac is also said to be conceding privately that his forces will not do well. In most districts, the Giscardists have rallied to his leadership of the center-right in an umbrella agreement under which the Giscardists and Gaullists are supporting most of each other's incumbents in the first round, without waiting for the runoffs on June 21. This means that a large number of center-right candidates can expect to be elected outright Sunday with an absolute majority of those voting but that the left should overtake them the following week when it combines it forces.

Chirac's power base as mayor of Paris is severely threatened. Questiaux said that the Socialists, as part of their ambitious program to lift the suffocating hold of Paris by strengthening local and regional powers, plan to introduce legislation to give each of the capital's 20 arrondissements (districts) an election council and budget, reducing the hold of the city's overall mayor.

"For the first time in French history," said the short, white-haired Questiaux last night at another of the nightly meet-the-candidate sessions she has been holding in the apartments of supporters, "the Left is going to have duration going for it. Mitterrand was elected for seven years and the National Assembly will be there for five. If we don't accomplish our programs this time, it will mean that we are incompetents."

In three weeks in office, the Socialists have already started carrying out campaign promises that could be honored without legislation -- raising the minimum wage 10 percent, raising old-age and family allowances, starting negotiations for a 35-hour work week, investigating some of the better-known affaires the Giscard government was accused of covering up and dropping many of the restrictive practices that civil libertarians had protested.

Most of the questions Questiaux fielded were about people's personal complaints over their pensions, dwellings, rents and jobs. "So much for ideology," muttered one of her campaign workers who normally deals with grand policy in a major ministry.

Questiaux responded to many of the questions about problems in the state low-rent housing projects that dot the 13th Arrondissement by urging 40 people jammed into the tiny living room to form tenants associations to make their demands known.So far, she said, her Cabinet colleague at the Housing Ministry has been disappointing. "This is the time to organize and to bring pressure to bear," she said. "Make yourselves heard. Make up lists of your grievances."

A graduate of the National Administration School, the recruitment ground for France's administrative elite, Questiaux is a member of the Socialist Party's officially constituted left wing. A lawyer who has specialized in social affairs, her post as National Solidarity minister gives her control over the vast social security system and most other welfare programs.

She played a central role at the congress of Epinay 10 years ago when Mitterrand merged his personal following into the Socialist Party and took control of the new organization.

Questiaux was named to Mitterrand's tiny transition team to deal with the outgoing Giscard government, Mitterrand singled out Questiaux to read his message to the 10th anniversary ceremony in Epinary yesterday "Don't let yourselves become heady with success, any more than you let yourselves be shaken by one setback or another," Mitterrand said.

Questiaux, in an interview, said she thought she had nearly converted a Washington audience to socialism last year. "Let me warn you. I'm very good at charming Americans," she said in the good English she learned from her Britisn mother.

Informed of a recent private comment by U.S. Secretary of State Alexander M. Haig Jr. that the Reagan administration does not know what the Mitterrand government plans, but "We'll kill them with kindness," Questiaux said, "We'll kill them back with kindness and a half."

But, she said, nothing could stoke the anti-Americanism of what she called "the people of the Left" more than for Washington to appear to dictate that Mitterrand cannot have Communists in his Cabinet. The Americans should learn to take the French as they are, she said, an expression of what she claims is her philosophy of not trying to convert people to new life styles.

Asked why she chose to battle Moreau rather than a conservative incumbent, of which there are 26 in Paris, Questiaux said she had spent her political career backing Communists in runoffs where the Socialists had to withdraw after being outvoted in the first round. "I'm tired of campaigning for Communists. I think we Socialists have earned the right to be the ones to represent leftist districts," she said.

The younger Moreau has built her career as a Communist around being faithful to a party line that has had many zigzags in recent times. "Me disagree with the party?" she was once quoted as saying. "May God preserve me from that."

A close friend recounted how Moreau joined the party at 16 after quitting school and being repeatedly told that she was too young to get a stenographer's job. So, the friend said, she knocked on the door of national Communist Party headquarters and asked the guard who came out, "At 16, are you too young to join the party?" Told no, she said, "Then I'm joining." The party, she has said, became her university.

In her speech in the schoolyard last night, Moreau stressed the personal services she has performed for the district. She spoke of the dedication of Communist deputies, noting that like her colleagues, she keeps only $1,100 of her monthly parliamentary salary of $4,000, giving the rest to the party. She spoke of a need for lots of Communist deputies, to keep the Socialist firmly on the Left.But, as she watched others speak for her, she looked distinctly glum.

Her friend recalled that in 1958, the Communists had been reduced to 10 seats. "We bounced back then," she said. "We can bounce back again."