Having accommodated Sen. Edward Kennedy's economic program to avert a blood bath at Madison Square Garden, President Carter remains in the grip of his persistent dilemma, typified by the challenge of Philadelphia District Attorney Edward Rendell.

"For once," active Kennedy backer Rendell told us, "we'll be able to test whether the platform means anything. We'll see whether Carter does anything about it." Rendell and other Kennedy Democrats are, in effect, giving the president an ultimatum: go to Congress with a big-spending anti-recession package before the election and really push it or we'll sit on our hands.

Carter's dilemma, therefore, is not confined to the limits of the Democratic National Convention or the fantasies of platform writing. It is a deadly, practical problem of harmonizing the passions of party activists with the desires of most Americans.

The dilemma was formulated convention eve by a veteran southern leader, a Carter whip here. The future of the Democratic Party depends on a move toward centrist enconomic doctrines and away from big government, a move hesitantly begun by Carter as president. Yet that move could not be so obvious at the convention that it would risk the total alienation of Teddy Kennedy.

Total alienation would be costly -- a delay of at least six weeks in activating liberal party workers, particularly in the crucial industrial states, to work for the president's reelection. Accordingly, throughout this convention, the president's men were torn over whether to play for delegates here or voters across the nation.

In the end, they had no chioce. "This convention is incredibly more liberal than the president," one senior White House aide told us. The overwhelming majority favored a New Deal-McGovern economic program unrelated to the business-oriented reindustrialization plan that Carter will soon unveil. The celebrated $12 billion anti-recession package would exceed $30 billion if all the platform promises were followed, say Carter economic advisers.

Thanks to the new rule requiring candidates to submit a written statement on the platform, the dilemma intensified rather than diminished once the Kennedy plank was adopted. The president's problem was not made easier by the attitude of such Kennedy activists as Paul Tully, one of the senator's best state coordinators. Tully contended that "a typical Carter waffle" on the plank "would be horrible" -- worse than outright rejection.

Outright rejection was in fact ounseled by Lt. Gove. Mario Cuomo of New York, a devoted Carter loyalist. Cuomo would have used the word "demagogic" if he had been writing Carter's reaction to the platform. The Carter administration economic policy-makers, led by Budget Director James McIntyre, wanted economic objections clearly stated.

They failed. The president's letter was written mainly by White House policy aide Stuart Eizenstat with advice from top Kennedy labor backer Douglas Fraser, United Auto Workers president. Eizenstate "sweated blood" over its wording. It is a minor masterpiece in avoiding confrontation, but may have inadvertently won the worst of two worlds. a

While sufficient to win Kennedy's endoresement, it did not win the hearts and minds of his followers (save in Michigan, where state Chairman Morely Winograd led 33 Kennedy delegates to Carter). "A fudge!" snarled state Attorney General Robert Abrams of New York. Other Kennedy delegates contended the evasiveness buttressed their conception of the president as a trimmer.

But more cautious Democrats were concerned that Republicans would capitalize on a high-spending, pro-abortion, anti-nuclear platform embodying Kennedy's philosophy. After Kennedy's flawlessly delivered reiteration of his old campaign themes, one of his floor whips -- former lieutenant governor Bill Doherty of South Dakota -- told Carter state chariman Ed Campbell of Iowa: "This is what the Democratic Party is all about."

Fighting Kennedy's economic plank, Gov. Jim Hunt of North Carolina told the convention what the polls reflect and Carter strategists believe: inflation, not unemployment, is still the No. 1 issue. "DonT doubt what our people are feeling. Do we want to try to tell them that fighting inflation isn't so important anymore?" The response was a few boos.

Late Tuesday evening, after Kennedy's emotionally draining performance, New York's Cuomo talked to a half-deserted Garden about the plight of the middel class. Two young delegates from Kentucky cheered, marooned in a sea of inattention. "It's about time," one said to the other.

The $25,000-a-year worker, angry at losing to inflation, received almost no attention in one of the century's longest conventions that lavished thousands of words on the unemployed, blacks, Hispanics, Indians, homosexuals, poor mothers with illegitimate babies and women's rights. The platform is theirs. Jimmy Carter would like to turn to battling Ronald Reagan for the middle class, but to do so, he must first satisfy Ed Rendell and his ultimatum.