D.C. Del. Walter E. Fauntroy, breaking ranks with his major financial supporters as well as the hierarchy of the city's Democratic establishment, will endorse Sen. Edward M. Kennedy for president at a press conference this morning.

The announcement, scheduled for 11 a.m. at Fauntroy's Capitol Hill office, follows by a week Major Marion Barry's endorsement of President Carter. It also follows a flurry of lobbying activity by the White House to lock up the support of every major party leader in the city in advance of the May 6 primary.

Fauntroy's decision to endorse Kennedy was reported by Kennedy's D.C. supporters, who had been notified. Reached at his congressional office last night, Fauntroy confirmed only that his presidential endorsement plans would be a topic of the press conference, and he was working on his statement. He would not comment further.

The announcement will give the Kennedy forces in the District an encouraging boost in the face of White House attempts to blunt the clear advantage Kennedy was perceived to have held in D.C. when he entered the presidential race last fall.

The Carter forces, by spending to much time and energy courting endoresements from D.C. officials, clearly raised the importance of those endorsements beyond that of previous presidential election contests here. Carter campaign officials reportedly hope to challenge Kennedy's strength among black voters with a major effort here on May 6.

Among Carter's supporters are Mayor Barr, City Council Chairman Arrington Dixon, Democratic Party chairman Robert B. Washington Jr. and four members of the City Council.

Along with Fauntroy, Kennedy counts three City Council members among his supporters in the city political leadership.

Fauntroy's decision may cost him the backing of some of his key financial supporters. A group of those supporters warned Fauntroy in a Jan. 10 meeting that he would have trouble raising funds for his reelection campaign if he endorsed Kennedy.