President Carter's campaign committee plans to step up its legal pressure on Sen. Edward M. Kennedy (D-mass.) next week by filing with the Federal Election Commission a new complaint against Kennedy's supporters, Carter's campaign manager, Tim Kraft, said yesterday.

The new action, which is to be in the form of an amendment to an earlier Carter complaint pending before the FEC, will ask that money spent by the various draft-Kennedy organizations around the country count against the state and national spending limitations that will apply to Kennedy's official campaign organization once it is formed.

Without such a ruling from the FEC, Kraft said, Kennedy will enjoy an unfair advantage over the president in the coming battle for the Democratic presidential nomination.

He said Kennedy will have the benefit of earlier spending by the draft committees -- for example, to purchase voter lists and print campaign material -- before his official campaign organization is charged with any spending against the limits imposed by the federal election law.

Kraft and other officials said the amended complaint will assert that, by his words and actions and those of his "agents," Kennedy has been a presidential candidate since late August or early September, and that all spending by the draft committees since then should count against his limits.

Kennedy, campaigning in Massachusetts, denied, as he has before, that he had anything to do with the various draft-Kennedy organizations.

Carter's political strategists say that one advantage they hold over Kennedy's aides is experience, gained in 1976, in dealing with the complex federal election and campaign financing laws. Their announcement of the new allegation against the Kennedy organization made clear that they intend to exploit that advantage.

Earlier this week, responding to the initial Carter complaint, the FEC issued a preliminary ruling that it had found reason to believe that draft-Kennedy officials may have violated federal campaign laws.

The original complaint alleged that the draft-Kennedy organizations are affilitated with one another, and not independent entities, and therefore are subject to the same requirements as other campaign organizations.

The FEC gave the draft-Kennedy officials 10 days to refute the FEC finding.

The new amendment to this complaint, to be filed next week, will center on spending. The issue is particularly important in some key early primary states such as New Hampshire, where the spending limit for the Feb. 26 primary is $264,600.