GOV. MICHAEL S. Dukakis's decision to run for the Democratic Party's presidential nomination will create a crisis in the Democratic Party, as two powerful sects within it try to dictate the party's future.

The sentimental dreamers in the party still look at the nomination of Geraldine Ferraro as a bold and worthwhile gamble. The cynical realists don't want to take risks any more. They just want to win.

The dreamers still think it's possible for a woman or a minority or an ethnic to be part of a winning ticket in 1988. The realists say a Mario Cuomo or Dukakis can't win because they can't do well in the swing regions of the nation--California and the South.

Democrats will have to choose soon which route they will take. The stakes are too big for the Democratic Party to display a schizophrenic front to the American voter for long. If the Democrats lose the next election it will mean 12 straight years--and possibly 16--of Republican control of the White House. The only Democratic President voters aged 18 to 30--huge cross-section of the electorate--will remember is Jimmy Carter.

WHERE ARE the 50.1 percent of the voters that Democrats can hope to capture in 1988? The West, except for California, is firmly in the Republican camp. Most of New England would vote Democratic even if the party candidate were Larry "Bud" Melman. Only Vermont, New Hampshire and Connecticut have a past history of voting against a reputable Democratic candidate--someone other that a minister's son from Minnesota.

If Robert Dole or George Bush receives the Republican nomination, Democrats will at best be able to split the Mid-West. The big industrial states of Ohio and Illinois will go Democratic, although a healthy economy could help the GOP. The farm states will vote Republican, as they have in the past four elections.

This leaves the two parties at a virtual dead heat--the Dems with 181 electoral college votes and the GOP with 190. Whoever captures California and the South wins.

The Democrats can't count on California, which supported Ford against Carter in 1976. That leaves just the South as the most important region to capture. And since Southern Blacks automatically vote Democrat, it means that Southern whites will have the crucial swing vote in 1988.

WHICH OF the probable Democratic candidates could do well in the white South? The presence of a woman or a minority on the ticket in 1988--even in the VP slot--will almost certainly throw the South whole-heartedly into the GOP camp.

Anyone with an ethnic image will likely find his candidacy stung by losses in the South--especially a Northeastern, liberal ethnic. Mario Cuorro knew this and decided not to run. Hopefully the Duke will learn this lesson also and help his party by dropping out of the race.

Besides being ethnocentric, Southern whites also form the most moderate portion of potential Democratic voters. To attract these voters Democrats have to adopt a middle-of-the-road image. In theory, Dukakis--with his pro-economic growth reputation--could pull this off. In reality, the Duke simply will be perceived in the South as an extreme liberal.

What the Democrats need is a Southern candidate on the ticket. Former Virginia Governor Chuck Robb or Georgia Senator Sam Nunn would draw droves of southern white voters.

To back up this strong appeal to Southern voters, the Democrats also need a centrist, non-ethnic non-New Englander on the ticket. Gary Hart is the only credible candidate who fits these categories.

HART WILL win big in the Northeast, although perhaps not as well as a New Englander would. He will also drain some of the Republican support in the West, particularly in Colorado and California. The presence of Nunn or Robb as a vice-presidential nominee will draw a majority of Southern whites.

Democrats will already be at a disadvantage in 1988. The Republicans will be the incumbent party during a period of decent economic growth. Reagan will still have enough popularity to lend significant support to the GOP nominee.

Democrats must unite now in support of a Hart-Nunn or Hart-Robb ticket. This means New Englanders must resist the temptation to support a favorite son and instead choose a candidate with the strongest chance of winning. Even if Massachusetts goes with the Duke, the rest of the country's Democrats must follow a different path.

It wouldn't be the first time the Bay State were out of step with the rest of the country. After all, if the American electorate followed the lead of Massachusetts in 1972, the winter of that year's election would have been George McGovern--perhaps the only person who would not have been an improvement over Richard Nixon.