Sen. Edward Kennedy's strategists have concluded it is unlikely the Massachusetts Democrat can capture the presidential nomination with an early knockout in President Carter's southern base -- and they have decided to prepare instead to fight a lengthy war of attrition.

In a pivotal decision, Kennedy officials say they have decided against mounting an all-out effort to win the Florida primary in March.

Financial resources must be conserved for the long haul, Kennedy campaign officials stressed in a series of interviews, and thus they have decided to make only a modest initial effort in Florida -- the state that represented Kennedy's best and perhaps only hope of cracking the foundation of southern support upon which Carter plans to build his effort to win reelection.

Kennedy officials said they would content themselves with the loser's share of the Florida delegates, which will be allotted proportionately bases on the results of the March 11 primary. They will look to later primaries outside the South, in states such as Illinois, New York and Pennsylvania, to defeat Carter.

A few months ago, when Kennedy held a two-to-one lead over Carter in the polls, a number of Kennedy advisers looked optimistically toward scoring an early knockout by defeating Carter in Florida. This, they reasoned, would demonstrate that the president had no chance to win the nomination because he could not hold his southern base.

Since then, however, the race between the two men has tightened considerably, as Carter's prospects have been boosted by public support for his handling of the crisis in Iran.

Kennedy, meanwhile, has gotten his election bid off to what even his campaign manager, Stephen Smith, concedes is marked by a "certain unevenness."

Carter advisers have built their campaign around the strategy of fighting a lengthy battle for the nomination, based on solid support in the South and the slow process of amassing delegates in primaries elsewhere.

Cater's campaign chairman, Robert Strauss, conceded last week that a Carter defeat by Kennedy in Florida could be the unmaking of the president.

"Obviously, losing Florida would hurt a lot more than losing any of the other early states," Strauss said.

But Kennedy campaign manager Smith, when asked about the prospects for a Kennedy victory in florida, said: "I don't think there's going to be an early knockout. I'm not looking at any single primary to be conclusive."

The Kennedy campaign planners have set spending targets with high, medium and low ranges for each state, Smith said, and in the next 10 days they will begin setting specific budget amounts. Florida's budget is likely, at least at the outset, to be modest, campaign officials said.

"We don't expect to carry the state," said one senior Kennedy official. "So it comes down to a question of how to target what we do spend. We'll have selective targeting."

The Kennedy campaign will be "targeting" three counties of the Gold Coast, the area of high-density transplanted northerners that runs along the Atlantic shore from Miami to Palm Beach. The campaign also will be targeting the Tampa area.

Kennedy officials have concluded that they can spend a minimal amount --perhaps $200,000 to $225,000 -- and still get perhaps a quarter of the 100 delegates who will be selected to attend the National Democratic Convention. d

"But the question is what do we do after that?" one campaign official pointed out. "For every $100,000 or $200,000 we add to that, what would we get? I don't know -- it's all just finger-in-the-air stuff."

Kennedy officials are leaving their final spending options open in Florida. Their decision could depend on how things go for Kennedy elsewhere. For example, after a late start, Kennedy aides now are putting together a good organization in Iowa.

If Kennedy scores impressively in his debate with President Carter in Iowa in January and then wins the Iowa caucuses, Kennedy officials say, they may decide to go all out for a victory in Florida, spending heavily for radio and television ads.

"The real spending decisions for Florida do not have to be made until mid-January," one official said. "That's when the media buys are made."

The Kennedy forces hafe been off to a slow, even stumbling, start in organizing Florida -- a sharp contrast to the Carter campaign, which has headquarters and paid staff operatives throughout the state.

So far, Kennedy has no headquarters and no paid staff in Florida. Kennedy officials say they will open a headquarters soon, probably in Tampa, with perhaps a couple of smaller satellite headquarters to serve the Dade, Broward and Palm Beach county areas.

"We could spend a fortune on headquarters alone," said one Kennedy aide. "But we don't want to do that. We just want to give people a place to call."

Much of what Kennedy is up against in Florida flows from the fact that Carter's campaign really has been going on for five years, Carter, his family, and his extended family of Georgians began working this state two years before the 1976 primary.

As president, Carter insistently has courted Florida.

A former governor (Reubin Askew), the former lieutenant governor, the former speaker of the state House and several other Florida political figures wound up with important jobs in the Carter administration, And the Florida Democratic establishment, beginning with Gov. Bob Graham is solidly behind Carter.

"Let's face it. Florida has done very well with the Carter administration," said Miami attorney Talbot (Sandy) D'Alemberte, a key Kennedy supporter. "Florida has never been treated better than now."

Florida also has treated Carter well. Carter officials say they have raised $700,000 in the state so far. Kennedy is coming to Miami to attend his first fund raiser in the state Tuesday. It is a major occasion, and he is bringing his wife, Joan, with him. Once Kennedy aides hoped to raise at least $200,000 at the dinner, but now they say it probably will bring in from $100,00 to $150,000.

Kennedy's supporters in Florida were in disarray after the defeat of the draft-Kennedy forces by Carter's organization in the politically meaningless caucus straw vote last October. The margin was relatively close based on total votes, but it produced for Carter an overwhelming three-to-one victory in the apportioning of delegates for a beauty contest balloting that then was held at the state Democratic convention in November.

But that contest this past fall is not a good yardstick for measuring the candidates' prospects in Florida. It had pitted the full power of the Carter White House and the Carter campaign organization against an officially unauthorized and unaffiliated draft-Kennedy effort.

Meanwhile, as the Kennedy priorities have shifted in florida, so have the Kennedy chiefs. National campaign official Harold Ickes originally was designated to organize the state. Ickes spent a couple of weeks in Florida, including a visit to St. Petersburg, where he represented Kennedy at the Carter-dominated state convention. He reported to national headquarters that virtually no one in Florida believes Kennedy can carry the state -- an assessment that proved pivotal in shaping the current strategy.

Ickes then transferred into New York to work on the Kennedy campaign there, and his place as taken by Tim Hanan, an old law school friend of Kennedy. Hanan's most recent political effort was serving as campaign manager for Abraham Beame's liseing effort to win reelection as mayor of New York City and whose most recent distinction is being the second Mobil Oil executive to join the campaign of the anti-big-oil senator from Massachusetts.

Kennedy officials concede that they have not had a good assessment of their strengths and weaknesses in Florida because there has been no campaign poll. This past week the Kennedy camp finally named a campaign pollster, Peter Hart, who has signed a contract to serve as official nonstaff surrveyor of public opinion.

One of Hart's first tasks, Hanan says, will be to do a poll in Florida to define potential areas of Kennedy strength. A month ago, a St.Petersburg Times newspaper poll showed Kennedy and Carter virtually even.

The decision to scale back the Florida effort apparently has been accepted by Kennedy's backers in the state. State Rep. George Sheldon of St. Petersburg, a leading supporter, explained:

"If we spent a fortune and tried to score the big knockout in florida and failed -- that would really hurt the Kennedy campaign." CAPTION: Picture, SEN. EDWARD M. KENNEDY . . . strategists foresee a lengthy war of attrition; Picture 2, In campaign speeches, he is at times hard-hitting, other times lackluster. AP