In Florida, demographics work for GOP. THE BATTLEGROUND. Migrants from north still come, most tend to vote Republican

If Republicans retain their United States Senate seat this year in Florida, some experts say that a major reason will be . . . air conditioning. Florida, long a die-hard Democratic state, has been transformed politically by an unending tide of new settlers. Many are Republicans -- from the farms of Iowa, the small towns of New England, the suburbs of New York and Philadelphia.

Sen. Paula Hawkins (R) of Florida, fighting for her political survival this year against a challenge by Democratic Gov. Bob Graham, wouldn't have a chance without these new migrants.

Stanley K. Smith, a demographer at the University of Florida, notes that the flood of humanity coming to Florida shows no sign of slowing. But he says that without modern, low-cost air conditioning to conquer Florida's humid, 95-degree summers, the boom would quickly fizzle.

Dr. Smith says the new settlers are arriving at a rate of 600,000 a year. That's enough to create a new Miami every seven months. Most of them appear to be Republicans.

The political revolution now under way here mirrors sweeping changes being felt across the American Sunbelt. It's a revolution that Republicans hope will put them in complete command in Washington, including the House of Representatives, by the early 1990s.

Florida is one of the best places to see what is happening. The state's demographics, like those of Texas, California, and other Sunbelt states, are being remade by a variety of forces:

Heavy migration from the North and Midwest. Sun, fun, and jobs are attracting new residents. The boom continues here and, to a lesser extent, in California but has slowed in Texas, hit by hard economic times.

Heavy foreign immigration, especially from Latin America. In Florida, the major immigrants are from Cuba, Nicaragua, and Haiti, but other Latin nations are also increasing. Most vote Republican.

The aging of the population. Florida's residents, now the nation's oldest on average, have elevated issues like social security to new levels of importance.

The baby boom. Even in Florida, the so-called baby-boom generation is huge, and it has moved toward the GOP, just as in other parts of the country.

One surprising fact: Most new arrivals in Florida are in the 15-to-24 and 25-to-34 age groups. They come looking for jobs and are adding to Florida's Republican strength. Only about 15 percent of new residents are 65 or older.

Floridians have always thrived on change. Indeed, they make their living on it. Construction of new houses, schools, universities, office buildings, and highways has been a pillar of the state's economy (along with tourism and agriculture) for decades. But even adaptable Floridians are having trouble adjusting to the pace of the last few years. Politicians, too, are uneasy. For example, demographers estimate that since Mrs. Hawkins last ran for the Senate in 1980, more than 30 percent of the voters are new. Since Mr. Graham first ran for governor in 1978, about 40 percent of the voters are new to the rolls. Many voters don't know either candidate well.

Republican strategists, however, would like to see the changes come even faster. They relish what they see.

When Hawkins first won her seat in 1980, Democrats had a 2.2-to-1 advantage in registered voters. Today that's been whittled to just 1.6-to-1. Changes have come even faster in Miami (Dade County), which has long been a Democratic bastion. When Hawkins ran in 1980, Democrats led in registrations by 485,000 to 138,000 (3.5 to 1). Today it's 418,000 to 231,000, or just 1.8 to 1.

Today, Miami Republicans -- about half of whom are Hispanic -- are one of Hawkins's secret weapons. She's set up a special Dade County campaign office. And the city has become a major source of Hawkins's campaign funds. The question for Hawkins is: Have all these changes come fast enough? Has Florida moved far enough toward the GOP to stay the course with a Reagan-Republican senator under challenge by a popular, moderate Democratic governor?

Graham, currently ahead about 10 points in the polls, strikes all the right chords for the ``old'' Florida. He's conservative enough (a strong backer of the death penalty) to keep the Dixiecrats happy in north and central Florida. But he's also moderate enough to appeal to the exiled, Northeastern liberals who fill the high-rises in southern Florida.

It's expected to be very close. But if they win, Republicans can thank not only Hawkins and President Reagan, they can also thank Willis Carrier, the inventor of the air conditioner.