The nine Common Market nations, who persistently deplore their collective inability to impose political weight on a world scale, now confront a sparkling opportunity.

Greece, Portugal and Spain - Western Europe's newest and shakiest democracies - art knocking at the community's door, convinced that Market membership will serve as a barrier to home-grown totalitarians.

For years, the nine members have been saying that the Mediterranean three would be welcome as soon as they deposed their dictators. But now that the three have done just that, the nine have suddenly discovered a multitude of problems.

The French and Italians are fretting over new competition to their producers of tomatoes, wine, peaches, olives, oranges and lemons. The Irsih are unhappy about sharing limited funds for poor regions with the even poorer Portugese and others. The West Germans, who pay most of the cost of elaborate subsidies to farmers and others, are groaning over the prospect of a bigger burden.

The three Benelux countries - Belgium, the Netherlands and Luxembourg - fear that enlarging the community from nine to 12 would make it harder to make decisions. They are frightened by their perennial nightmare: That a directorate of Britain, West Germany and France would run things without them.

Even the Common Market bureaucrats are uneasy. They would not like dividing their plush jobs up among three more countries.

During the winter and spring, the grumbling from all these quarters became so loud it appeared to threaten the community's great political opportunity. But in recent days, the Market members have begun to focus on the damgae they could do to western democracy dna defense if they kept the three out. It now seems clear that - after some tortuous bargaining - the door will be opened to the Mediterranean trio.

There is unusual agreement in Brussels, among the applicants and the member states alike, on how entry will strengthen deomcratic forces in Portugal, Greece and Spain. The community's rules - at least on paper - call for the free movement of workers and capital among members and, in some cases, reliance on market forces to determine output and price.

A totalitarian state has trouble operating such a system, as Chile is now demonstrating. Authoritarians demand much tighter control.

The Market describes itself as a grouping of democratic states, so a member that turned totalitarian would risk isolation from the markets and money that membership brings.

The trading system of the community carries not only the goods but alos the ideas of members from one to another. And finally, an economic crisis that might encourage a coup could be alleviated by aid from west Germany and other alarmed community partners.

A West German diplomat here brushed aside the possible financial burden. Recalling recent fears that an anarchic Portugal would be taken over by leftist extremists, he said: "Think what we would have paid two years ago to have the Portugal that we have today."

In some ways the Italians and French, who have most to fear fomr the competition of Mediterranean farmers, also have the greatest political stake in the membership of the three. Rightist terrorists in both countries were often supported in the past by etxremists in power of close to it in Spain, Portugal and Greece.

There is a common view in Brussels that the United States and other NATO countries have a strong internal in seeing that the three join the nine and are not kept isolated. There have been hints from Madrid that Spain's enthusiasm for joining NATO might be sharply diminished if Madrid were kept out of the Market. The Spanish have suggested that the defense costs imposed by NATO must by balanced by community membership.

Somewhat surprisingly, many diplomats here hope that Washington will encourage the nine to take the three in. Although community members, especially France, have frequently complained that the United States was trying to dominate the market with high-handed tactics, the three potential members and several of the nine members want the United States to encourage enlargement.

Expansion won't happen quickly, he Greeks, with strong French backing, applied for membership 16 months ago. A stately diplomatic minuet has ensued, and genuine bargaining may finally start in September, it is expected that Athens will be admited sometime in 1979.

The Greek ambassador to the nine here, Stephane Stathatos, says, "This is a challenge for the nine. They must show the outside world their principles are not dead words, that they are not a club of the rich. The frustration of the Greek people would be insupportable" if the application failed.

Once the Greeks are in, it will be hard to deny the others. Portugal applied this spring and Spain is expected to put in its bid at the end of the summer.

A Spanish application is not certain, however. Some of the country's leading economists insist that Spanish industry must keep its present protection and subsidies, and these violate Market rules. However, all major parties in Spain have endorsed entry and Spanish officials here are persuaded that membership is coming.

The negotiating minuet for Madrid will be the stateliest of all. Spanish citrus-growers can undersell the French and Italians by as much as 40 per cent. Jobless laborers from smaller Portugal and Greece pose only a limited problem. But the 35 million in Spain could send out a fresh wave of workers just as the nine are wrestling with stubborn employment problems of their own. The expectation here is that the bargaining over Spain's bid could take four years.

The French, with some support from the Italians, have in typical fashion used the enlargement issue to press for added protection for their own Meditterranean growers.

The Market now is burdened with an elaborate system of price supports and subsidies that raise food costs and create lakes of wine and mountains of surplus butter and beef.

But this system provides much less support for Mediterranean growers than for those in the north, and the French want it extended. From time to time, the French say that bargaining with the three can't even start until the added protection is in effect.

France's partners are convinced that this is a ploy, linked to next year's parliamentary election to impress voters in the Midi.

At least some in the Market bureaucracy think that the French are brandishing a two-edged sword: that opening up the issue of protection for southern growers could open up for possible reform the entire jerrybuilt structure that the nine call their "common agricultural policy."

In the end, however, almost everyone here predicts that the French and Italian growers will somehow be bought off and that the three new democracies will in time take their place in the community.