The Federal Election Commission in a preliminary ruling, has found "reason to believe" draft-Kennedy officials "may have violated" federal campaign laws as the Carter-Mondale committee recently contended, according to sources in the draft-Kennedy movement.

The FEC has notified officials of the various draft-Kennedy groups of its finding in a certified letter, the sources said. The commission has given the draft-Kennedy groups 10 days to provide information to refute the initial finding.

According to the letters, the commission has found "reason to believe" the various draft-Kennedy committees which claim to be independent, are affiliated. It also has found "reason to believe" that the committees, because they are affiliated, received contributions from the political unit of the International Association of Machinists and Aerospace Workers which violate the $5,000 limit for campaign contributions from one political committee to another.

And it has found "reason to believe" some of the national draft-Kennedy groups failed to register with the FEC and disclose contributions and expenditures as required.

All these contentions were raised in the complaint filed recently before the FEC by the Carter-Mondale campaign committee. Each of the contentions also has been denied by officials from the draft-Kennedy organizations. The draft-Kennedy officials contacted yesterday said they will spell out their denials in detail for the FEC within the next 10 days.

According to an FEC source, the commission's finding of "reason to believe" is the first of a three-step process before any final ruling could be made and penalties and fines could be assessed. The commission could not have proceeded with its investigation, or sought further information, without having made this preliminary finding, the source said.

The FEC letters were mailed to organizations listed as "respondents" in the original Carter-Mondale complaint, including the International Association of Machinists and Aerospace Wokers, the National Call for Kennedy (which is headed by machinists union president William Winpisinger) and various draft-Kennedy committees in Florida, Iowa, Illinois, Minnesota and New Hampshire.

According to the U.S. Code, a guilty finding could result in penalties including a fine not to exceed $25,000 or an amount equal to triple the total of the illegal contributions. The law also provides a one-year prison term for officials deemed liable.

Meanwhile, because turnabout is fair politics, the machinists union, in addition to denying the allegations, has also asked the FEC to investigate the Carter-Mondale campaign. In an Oct. 11 letter to the commission, the union reminded the FEC of a statutory provision that says the FEC should also investigate any candidate or committee which makes an allegation against an opponent to the commission.

Specifically, the machinists have asked the FEC to investigate whether the federal government has been paying expenses and salaries of administration officials who have been flying to Florida and Iowa and elsewhere on government business -- but making campaign appearances as well. The administration has maintained that such expenditures for trips of mixed purposes of government and politics are divided proportionally.

The original Carter-Mondale campaign committee asked the FEC to:

Find that the various draft-Kennedy groups were affiliated and to prohibit them from acting in the future as if they are separate.

Require groups that had not been registered and had not met contribution reporting requirements to do so.

Prohibit further contributions to the various draft-Kennedy groups which, when totaled, would exceed the limits allowed for contributions to a single campaign organization.

Carter-Mondale campaign committee counsel Tim Smith declined comment on the preliminary finding of the FEC. Sen Edward M. Kennedy and his spokesmen have repeatedly dissociated themselves from the various draft-Kennedy movements and have disavowed their efforts. Kennedy is expected to disclose formation of his exploratory campaign committee Friday.