As Democrats convene, Mondale's challenge is to hold party together

Lights! Cameras! Unity! That's the script Walter Mondale wants to see during the next four days as the Democratic Party again goes on millions of screens in its quadrennial TV extravaganza.

The key word is ''unity.'' But it won't be easy. As delegates poured into this city, dissention was already growing with reports that Mr. Mondale had chosen a famous name from the Jimmy Carter era - former White House budget director Bert Lance of Georgia - to play a key role in his campaign.

Some Democrats, especially from the West, were shocked at initial news stories that the current chairman of the Democratic Party, Los Angeles attorney Charles Manatt, would be replaced by Mr. Lance. Later, Mondale officials backed away, saying that for the time being, Lance would be ''general chairman'' of the campaign - with the job of party chair still to be decided.

But Mondale's unity problems run much deeper than one controversial appointment. The Democratic Party is one of striking diversity and wide-ranging philosophies. Often its members seem as interested in battling each other as in fighting the GOP.

As the national convention convenes here today, the colorful Democratic mosaic can be seen all over this city - in parades in the streets, speeches in the parks, and on the convention floor itself.

The city is alive with politics. At the Hyatt Regency Hotel, black female delegates caucus before the convention opens. American Indian delegates plot strategy at the Fairmont. Thousands of homosexuals and unionists march in the streets. Senior citizens rally at Longshoreman's Hall. Feminists devise political plans at the Hilton. There are nearly 200 meetings, rallies, and press conferences outside the Moscone Center.

Walter Mondale's task will be to meld this supercharged army of Jews and blacks, Roman Catholics and Southern whites, feminists and conservative Protestants, into a cohesive fighting force. Only with almost everyone aboard can Mondale hope to wrest power from a popular president during prosperous times.

It was a similar coalition, first put together under Franklin D. Roosevelt, that from the 1930s to the '60s put Democrats in the White House for 28 out of 36 years.

Mr. Mondale wants to renew that tradition. From the very start of his campaign, he has used a building-block process to put together his support. He began with the big industrial unions. He courted teachers, blacks, Hispanics, Jews, big-city mayors, and Southerners.

Yet for many reasons - some economic, some demographic, some historic - the Mondale campaign this year could become a last hurrah for the old Rooseveltian strategy.

Sen. Gary Hart challenged Mondale by appealing to a new breed of younger, upscale voters - and almost won. Conservative Democrats, represented by such fallen candidates as Sen. John Glenn and Sen. Ernest Hollings, have little representation here.

Nationally, tensions within the party have grown. Anger and alienation has mounted between Jewish and black Democrats. Southern whites, strong Roosevelt supporters, now gravitate toward the Republicans. Northern blue-collar ethnics, many with roots in Soviet-dominated Eastern Europe, prefer Mr. Reagan's tough brand of anti-communism. The abortion issue, troublesome to many Roman Catholics , erodes support for liberal Democrats in the North.

Political experts are divided on whether the party is undergoing a radical change. Some, such as David Chagall, publisher of ''Inside Campaigning,'' say that we are witnessing the breakup of what is left of old party coalition. Others, such as political scientist Norman Ornstein, suggest that rumors of its imminent demise are greatly exaggerated.

Mr. Chagall sees in 1984 ''an historic change in the party alignments.'' He predicts more and more of the old loyalists leaving the party: Southern whites, union members, Jews. He sees a Democratic Party that is more and more under the control of liberal activists, and veering away from Middle America, where President Reagan draws much of his strength.

Dr. Ornstein disputes that, but does concede the Democrats have a problem - specifically ''an image problem.''

The party, Ornstein suggests, is seen by many voters as being out of whack with ''traditional values'' - family, religion, morality, honesty, hard work, free enterprise, patriotism. This image problem, he says, can be dealt with through effective leadership.

As an example, he offers the current governor of New York, Mario Cuomo, who ''manages to evoke all of Democratic liberalism in the context of traditional family values.'' Governor Cuomo's keynote speech tonight, says Ornstein, could be a vivid example of how the Humpty-Dumpty of the Roosevelt coalition could again be pieced together.

Even Mondale's choice of Rep. Geraldine A. Ferraro as his running mate, however, is seen by some analysts as a sign of Democratic weakness. Prof. Stephen J. Wayne of George Washington University calls it an act of desperation.

The old North-South alignments just aren't working for the Democrats any more , Dr. Wayne says. If the coalition were alive and well, Mondale would almost certainly have gone toward the South for his vice-president.

The 3,944 delegates and 1,313 alternates who assemble at Moscone this afternoon are well aware that since 1968, they've won only one presidential election - and that by a slim margin.

In the next few days, Mondale's clear task is to strengthen party unity. Ms. Ferraro will help him with feminists. Mondale now must reassure male Democrats and Southerners, who may feel ignored, despite the Lance appointment.

His long-range task: to convince voters that the party stands for middle-class values, family, and patriotism.