The French Communist Party is showing firm determination to risk defeat in the approaching parliamentary elections rather than give ground in its struggle for dominance of the French left.

The Communists are saying that they do not intend to step aside in favor of Socialist candidates in the crucial runoff elections to be held March 19. Should the leftist vote be split at that time it is unlikely that the Socialist and Communists could win enough seats to control the new parliament.

The alternative would be continued rule by France's center-right coalition.

In an interview on state television Wednesday night, Socialist Party leader Francois Mitterrand said, "The Communist Party can make the left lose the elections."

When the Communists first started suggesting that they might not go along with Mitterrand's demand that they respect the same "republican discipline" that has been the rule between leftist parties in every election since 1972, Socialist leaders said that the threat was a monetary tactic. But in the past 24 hours the Socialists have started talking as if they take the threat seriously.

It is generally conceded that the left now has the votes to win the election provided the leftist parties join forces for the runoffs which come one week after the first round of voting.

When the Communists took their new line last week, the Communist newspaper L'Humanite rejected "republican discipline." Editor Rene Andrieu wrote, "Too often non-Communist left profited from our automatic withdrawals which were not always reciprocated and then took advantage to follow rightist policies after having pledged leftist ones."

Part of the reason the Socialists did not initially take the Communist threat seriously was apparently that Communist leader Georges Marchais has made so many zigzags in the past two months that the latest line could be dismissed as yet another tactic.

"I have to go find out what Marchais says our position is today," joked a normally well-informed Communist on the way to a meeting.

Underlying the seeming inconsistencies, however, has been one constant: a search for a way to change the balance of forces on the left in favor of the Communists.

When the Socialists and Communists first signed their joint program in 1972, the Communists were the dominant party on the left. Since then, however, the Socialists Party's commitment to be a party of the left after years of flirtation with the moderate parties of the center gave it a new dynamism, to the point that it now consistently polls almost 30 percent, while the Communists get about 20.

The two parties split in late September over Communist demands that the 1972 agreement be updated to include a far more sweeping program of nationalizations of companies.

At the time the Socialists also gave the impression that they would be happy to see the joint program lapse. They proposed that a compromise number of major companies be nationalized, but that their subsidiary divisions be left in private hands, an approach the Communist ridiculed.

In October, Communist leaders were saying privately that the worst possible outcome from their viewpoint would be a victory of the left with no jointly agreed program. They said that it would create a situation in which the Socialists could dominate a new government with the Communists, as junior partners, stripped of their leverage to demand a sufficiently leftist program.

More recently the Communist have been saying that mitterhand wants to come to power as head of a minority Socialist government excluding the Communists.

The Communists keep harking back to something Mitterrand reportedly told a closed-door meeting of the Socialist International in Denmark a few years ago: that his objective is to reduce the Communist Party to 15 percent of the French electorate or even less.

Mitterrand has never denied having saying it.

Most independent analysts in France have concluded that this comment is the heart of the Socialist-Communist debate. The Communists are perceived to be struggling to maintain their long-term future against a growing Socialist Party that seems to be running off with the Communists' traditional role as the party of the French working class.

When the Socialists and Communists first split in September, a number of commentators claimed to see "hand of Moscow," suggesting that the rupture was made on Soviet orders.

It is now generally conceded, however, that the French Communists did not need to be told by Moscow where their long-term interests lay. In fact, Soviet diplomats in Paris are broadly hinting to westerners and French Socialists that they are unhappy with the Communists for scrapping the alliance.

Meanwhile, a sense of relief seems to have entered into conversations with government leaders and high officials as they contemplate the prospect that they will remain in power. They express no illusions, however, that life will be easy for them if they stay.

Georges Seguy, the Communist leader of the General Labor Confederation, the country's largest trade union grouping, has already said that if the government wins re-election, there will be a major wave of strikes.

Inside the left, there would also undoubtedly be a long period of mutual recrimination. Socialists say that it would take 15 or 20 years before the two parties could re-establish mutual trust.