Long known as a frivolous talking shop, where politicians would skip debates to linger at lunch over extra bottles of Alsatian wine or take breezy cruises down the Rhine River, the European Parliament is bracing itself to assume a new role as the elected voice of the 260 million citizens of Common Market countries.

After years of bitter haggling that nearly split the ruling coalitions in France and Britain voters in the nine nations of the European Economic Community are scheduled to vote late next spring to choose 410 representatives for an enlarged European Parliament. Those elected deputies will replace the current 198 members who are appointed by national legislatures.

Advocates of European unity feel that direct elections will serve as a catalyst to revive the dormant vision of "a sort of United States of Europe," as once described by Winston Churchill. Opponents label the vote "a surrenter of national sovereignty."

In spite of the fierce polemics, preparations for the elections have already started to affect the politics in member countries.

Increasingly, political parties are tending to galvanize along ideological rather than national lines. European Socialist, who form the biggest voting bloc in the European Parliament, have established close ties across national borders, provoking the Christian Democrats and the liberals to band together in a similar way.

After incurring the wrath of Moscow for their maverick ways, European Communist parties have held several strategy sessions, seeking to forge common policies on some issues. The Frech Communists who for years maintained an alliance with the Gaullists in staunch opposition to the European Parliament, finally accepted the principle of European elections after some subtle coaxing by their Italian comrades.

Italian Communists have always taken their work in the European Parliament seriously, perceiving the forum as an important tool "to help Europe develop, through democratic institutions, into a neutral bloc distinct from the United States and the Soviet Union," Giogio Amendola, chairman of the European Parliament's Communist wing, said recently.

The deputies of Strasbourg are seated in the assembly hall according to party affiliation, a tradition that has fostered political cooperation across national boundaries.

Still, apart from its continental scope, the European Parliament wields little political influence. It possesses only limited powers to approve the Common Market budget and the authority - never yet exercised - to sack the EEC Executive Commission.

All key decisions in Community matters are taken by unanimous vote of the foreign ministers of EEC nations. The European Parliament is often consulted, but its views rarely sway the views of the ministers.

Once direct elections are held, the Strasbourg deputies are hopeful that the Parliament's new status as the sole, Market-wide voice of popular consent will lend moral weight to the quest for full legislative powers.

For the time being the Parliament can do little but engage in humdrum debates. Many of the exchanges involve such topics as the relative merits of Italian versus French tomatoes or the contents of English ice cream. Delays in translating the six working languages prevent spontaneous remarks from enlivening the tedious speeches.

In fact, The European Parliament does not even have a permanent home, although it is often identified with Strasbourg. Like a wandering circus, its members must shuttle one week a month from their national capitals to Luxembourg, Brussels or Strasbourg, depending on the season or issues to be discussed. The itinerant formula was set up to resolve the touchy question of how to share the community's institutions.

The members are trying to center the entire base of operations close to EEC headquarters in Brussels. The move would lessen travel burdens and trim the European Parliament's budget, which now exceeds more than $80 million a year - more than twice that of most national legislatures. France and tiny Luxembourg, however, are relucant to part with whatever prestige they derive as part-time hosts.

For all its weaknesses, the European Parliament's drive toward direct elections and its search for more political influence have elicited an active interest from the U.S. Congress.

Since the turn of the decade, the two assembies have exchanged delegations twice a year to promote U.S. European cooperation on legislative matters of mutual concern. One subject now under study is a joint code of conduct for multinational businesses operating in the United States and the Common Market.

"It took the U.S. Senate over 100 years to get direct elections, but European Parliament will have theirs after only 20 years," Rep. Sam Gibbons (D-Fla.) said during a recent swing through Europe.

Such eminent European politicians as former West German Chancellor Willy Brandt and French Socialist leader Francois Mitterrand plan to run in next spring's elections.

Even with such political heavy-weights in the newly enlarged assembly, the European Parliament faces a treacherous battle in its effort to acquire real legislative power. Any such attempt will encounter stiff resistance in many national parliaments. French Gaullists and anti-European factions in the British Labor Party have vowed to block any mores they feel might jeopardize national sovereignty.

"There is no question of taking away national powers through direct elections." Guy Van Haeverbeke, a spokesman for the European Parliament, said. "The European Parliament will operate within the Common Market's founding Treaty of Rome, which all member countries have signed and which calls for an elected European Assembly.

"We hope that European MPs will be able to take decisions in a European context, but final authority for national issues will still lie with member states' parliaments."

Divided roughly according to SEC populations the 410 seats will be alloted in the following manner: Britain, France, West Germany and Italy, 81 seats each; the Netherlands, 25; Belgium, 24; Denmark, 16; Ireland, 15; and Luxembourg, 6.

The prospective new members of the Common Market - Greece, Spain and Portugal - have requested that they, too, be included in the European elections, in anticipation of the day when they join the community, but it appears that the three Mediterranean countries will only be granted observer status until they join.

While all EEC states have ratified the European elections bill, or expect to in the coming months, a number of delicate issues still have to be settled before the vote can take place.

Several countries say the plan for proportional voting clashes with their traditional electoral methods and are seeking compromise arrangements. Another problem that still needs to be ironed out is whether countries will permit their European representatives to continue to serve in national legislatures. If not allowed to keep their old constituencies, many important leaders such as Mitterrand would probably abandon plans to run for the European Parliament.