President Carter's campaign strategists, confident of having the presidential nomination sealed next week, have postponed the party's traditionally contentious platform hearings -- hoping Sen. Edward M. Kennedy will decide in the interval against forcing a costly convention floor fight.

Officially, the hearings in Washington were postponed until mid-June by Democratic Party Chairman John White.

But the idea to postpone them was born in a meeting of Carter aides earlier with the president's campaign chairman, Robert Strauss, according to senior Carter campaign officials. Strauss then relayed the idea to the party chairman, who is nominally neutral but personally a Carter supporter. And White ordered the hearings delayed.

"John was quick to see the wisdom of the suggestion and he was quick to adopt it as his own," said one Carter campaign official.

The Democratic platform hearings were to have convened here next Thursday, just two days after the primary election finale. Now the hearings will be held June 12-14.

As the Carter strategists see it, the one-week delay could be crucial to the psychology of presidential politics, 1980. The Carter officials say that, according to their calculations, the president will clinch the nomination -- and in fact, have a sizable surplus of delegates -- in the eight primaries on June 3, even though they concede Kennedy is likely to win in California and perhaps New Jersey. They say they are confident Carter will capture the Ohio primary contest, and it is there that they are making their greatest effort and spending the bulk of their campaign funds.

The Carter campaign will spend about $400,000 in Ohio on radio and television advertising, according to officials.

In contrast, the Carter campaign had budgeted just $91,000 for media advertising in California -- all with stations in the Los Angeles area. In the last few days, the campaign has added another $50,000 for media advertising in the San Francisco Bay area.

The Carter campaign had not planned any media advertising in New Jersey, which has no major television stations. But Carter officials have now decided to spend. $90,000 which also serve New Jersey. Recent polls of Carter's public opinion analyst Patrick Caddell show Carter trailing Kennedy by 4 or 5 percentage points in New Jersey -- well within striking distance and by a somewhat narrower margin in California, but with a high percentage of undecided voters.

Carter now has 1,662 delegates, according to his delegate counter, Thomas Donilon. This is just four short of the 1,666 required to win nomination. Kennedy has 861 delegates, according to the Carter campaign's figures, with California Gov. Edmund G. (Jerry) Brown Jr., having one and 89 officially uncommitted. The Carter delegate count includes projections that apportion delegates in caucus states that have begun, but not completed, their selection process. Other counts show him farther from the 1,666 figure, but likely to pass it next week.

Carter strategists estimate the president will come out of the Tuesday primaries with about 1,950 delegates to Kennedy's 1,350, even if the Massachusetts senator wins decisively in California and narrowly in New Jersey.

This surplus of almost 300 delegates -- all committed to vote for Carter on the first convention ballot -- will be seen as an insurmountable margin, the Carter officials believe.

The Carter officials hope that following the Tuesday primaries, a number of Kennedy's supporters will urge him to find a graceful way out of the contest for the nomination and not force a floor fight at the convention.

Kennedy advisers have spoken of the possibility of attempting at the convention to change the party rule that binds delegates to vote on the first ballot for the candidate to whom they were committed by their state primary and caucus results. A change would have to be approved by a majority of the delegates, and Carter officials voice confidence they can hold their delegates and defeat any such effort.

In a recent meeting in Strauss' office, Carter campaign research director Martin Franks noted the platform hearings were scheduled to begin just after the last round of primaries. And he is said to have pointed out that the hearings could refuel the frustrations and antagonisms of the Kennedy supporters at a time Kennedy might be deciding whether to abandon his all-out fight for the nomination. So Strauss concluded it would be best to have the hearings postponed.

Top Carter campaign officials, including Strauss and campaign manager Tim Kraft, have told leading Democrats that a Kennedy effort to carry the fight through the summer will tie down Democratic Party fund-raising efforts through mid-August. They maintained that would give a large and lucrative head start to Republicans, who no longer are preoccupied with the business of selecting a presidential nominee.