Sen. Edward M. Kennedy (D-Mass.) was reported last night to have decided to take himself out of the 1984 presidential race. Widespread but unconfirmed reports were that he will make the announcement today.

Kennedy was not available for comment last night, and staff members said they could say nothing about the subject until today.

Junior staff members said Kennedy was meeting late yesterday with top aides presumably to prepare the announcement. Shortly after midnight, six Kennedy aides emerged from the senator's home in McLean but made no comment.

But reports swept Capitol Hill and Democratic Party circles that Kennedy, after a Thanksgiving weekend family gathering on Cape Cod, had decided to yield to his children's pleas that he not run in 1984.

The reports came from various sources involved with Kennedy's recent reelection campaign and earlier Kennedy races, and from friends of the senator and his family. None could be confirmed, however.

Kennedy, who failed in his challenge to President Carter's renomination in 1980, has been leading in early polls of Democrats' preference for the 1984 prize.

But most political observers think he would face a serious challenge from former vice president Walter F. Mondale, Sens. John Glenn (Ohio), Gary Hart (Colo.), Ernest F. Hollings (S.C.) and Alan Cranston (Calif.), former Florida governor Reubin Askew and perhaps others.

Kennedy, 50, won his fourth full term last month. Shortly after the election, it was announced that he and his wife, Joan, legally separated for two years, would complete formal divorce proceedings soon.

Joan Kennedy was reportedly present last week at the Kennedy compound in Hyannisport, Mass., when Kennedy's three children, whose ages range from 22 to 15, and other family members discussed the senator's political plans with him. Sources described the children as "adamant" in opposing another presidential race while they are still in school, but reportedly they did not rule it out in the future.

The initial reaction of Democrats centered on the impact of Kennedy's reported decision on Mondale. Most saw it as a mixed blessing for the Minnesotan, the only other contender who has been through a presidential campaign and the runner-up to Kennedy in most polls.

On the face of it, Kennedy's withdrawal should improve Mondale's chances of gaining the endorsement of the AFL-CIO a year from now. Both have strong backing among union leaders but federation rules require a two-thirds vote for endorsement. Without Kennedy, Mondale would seem by far the likeliest to receive that support.

But, as many were quick to note, Kennedy's withdrawal would also expose Mondale to the hazards of being the front-runner for the nomination and maintaining that position -- against all comers -- for the nearly 20 months to convention.

Other speculation centered on the possibility of additional contenders entering the race. Rep. Morris K. Udall of Arizona, runner-up to Carter in the 1976 primaries, has hinted he might run if Kennedy did not, and others might reassess their positions if Kennedy were out of the picture.

The drama of Kennedy's decision is part of a familiar pattern. Ever since President Kennedy was assassinated in 1963 and Robert F. Kennedy was killed while seeking the 1968 nomination, Edward Kennedy has been under quadrennial pressure to put his name on the ticket.

He cut off a move to draft him in his brother's place in 1968. In 1972, he filed affidavits of non-candidacy to remove his name from primary ballots and then spurned George McGovern's plea that he become the vice presidential candidate.

In September, 1974, well before the 1976 campaign began, Kennedy once again removed himself from consideration in a statement that left no room for evasion or reconsideration.

In 1980, the Carter forces successfully exploited ads questioning Kennedy's character, with on-the-street interviews with citizens saying "I just can't trust him." The ads played on doubts about Kennedy's veracity that pollsters have found persisting ever since the accident at Chappaquiddick Island in 1969, when a car Kennedy was driving went off a bridge and his companion, Mary Jo Kopechne, drowned.