President Carter and his supporters in Washington's business community are using the federal treasury, patronage jobs and the lure of campaign contributions to lock up endorsements from virtually every top elected leader of the Democratic Party in the District of Columbia.

Mayor Marion Barry, whose recommendations for key federal jobs have been delayed by the White House because of Barry's low-profile public support for Carter's reelection, formally endorsed the president yesterday at a full-blown press conference attended by First Lady Rosalynn Carter.

Barry's announcement came only a few days after Del. Walter E. Fauntroy, who has flirted with the idea of backing Sen. Edward M. Kennedy (D-Mass.), was told by local businessmen to either endorse Carter or not expect money from them for his own reelection campaign.

The Carter forces have already obtained the support of the District's Democratic Party chairman, the chairman of the City Council and four council members. Fauntroy is the only major elected Democrat in the District who is still publicly undecided.

As a result, Carter, who only several months ago was generally considered to be a sure loser to Kennedy in the May 6 District Democratic primary, now appears in a position to make at least a stronger showing -- one that would establish his strength among black voters who were decisive to his election in 1976.

White House sources said yesterday that Barry's formal endorsement could expedite selection of a new chairman for the Pennsylvania Avenue Development Corp., which has been without a chairman for more than two months. Part of the delay, sources have said, was due to public uncertainty about the support for Carter by Barry and his nominee for the post, lawyer Max N. Berry. Berry attended yesterday's press conference and now says he supports Carter.

Already, sources said, the White House had begun to process some of Barry's recommendations for nearly 1,500 jobs to be filled in connection with the taking of the census this year.

Over the last several months, the Carter administration has courted Barry with million of dollars in federal grants, much of which has been doled out through the White House office that coordinates relations with state and local governments. The District has received federal funds for projects ranging from nutritional aid for the elderly and rent subsidies for the poor to urban renewal and energy conservation.

At yesterday's press conference, however, Barry said that his support was not linked to the additional resources granted the city. "I'm not looking for any specific handouts of any kind," the mayor said. "It would probably take all the oil (money) in Saudi Arabia and Iran combined to buy my vote."

Money is precisely what Fauntroy, who will be up for nomination to a sixth term in the May 6 election, was looking for Jan. 10 when he met with 18 key business leaders in the board room of the National Savings & Trust Co. Fauntroy had called the meeting to help raise $18,000 for a Feb. 15 birthday party/fund-raiser.

According to some who attended the meeting, the businessmen told Fauntroy that it would be difficult for them to fulfill their $1,000 pledges if Fauntroy supports Kennedy. Many local businessmen oppose Kennedy because of his position on national tax reform and health insurance.

"(Kennedy's) what you would call a very liberal Democrat in most businessmen's minds," one who attended the fund-raiser said privately yesterday. "We look to him as a guy who has never worked a day in his life. He has inherited money and never worked to improve it. He's got a very liberal approach to spending other people's money."

Fauntroy, who could not be reached for comment yesterday, is said to have told those at the meeting that he would give them an answer on his presidential preference within 24 hours. He has yet to do so. Asked by a reporter Saturday night who he would be endorsing for president, Fauntroy, a Baptist minister, responded with a smile, "I'm still praying on it."

Among those who attended the meeting were National Savings & Trust president Joseph H. Riley; real estate executives Raymond J. Howar and Foster Shannon; Thomas J. Owen of Perpetual Federal Savings & Loan Association (who is competing with Berry for the PADC post) and lawyers Joseph Tydings and R. Robert Linowes.

Fauntroy had been considered a likely supporter of Carter. He has close political ties with District Democratic Chairman Robert B. Washington Jr. and City Council Chairman Arrington Dixon, both of whom have already backed the president. Fauntroy is also a close personal friend of former ambassador to the United Nations Andrew Young and Coretta Scott King, both of whom are also supporting Carter.

But in the past few weeks, bolstered by advice that he could break with Barry by supporting Kennedy and still not suffer in the May 6 primary, Fauntroy has toyed with a Kennedy endorsement. Bob Bates, a key Kennedy campaign aide, has taken the lead in trying to coax such an endorsement.

"Bob Bates spends half his time up here," one Fauntroy aide said, "and Steve Smith (Kennedy's brother-in-law and campaign manager) has called here many, many times."

The Carter forces have not been passive.

On Jan. 3, when Fauntroy attended the White House signing of a Metro funding bill, the president and Vice President Mondale met privately with him to solicit his endorsement, Fauntroy told his staff.

Upon returning to his Capitol Hill office, Fauntroy told one staffer, "I'm just very troubled. You just don't dump someone after you've developed this kind of relationship," according to a Fauntroy aide.

Barry, at his press conference yesterday, pledged to actively campaign for the president. He noted that his announcement was made two years to the date after he had formally announced his own campaign for the Democratic nomination for mayor.

"Just as my decision to run for mayor was raised on my vision of what could be done to provide the best government for the citizens of the District," the mayor said, "my stand today is based on what is best for the people of the District of Columbia, what is best for black people and other minorities and what is best for America."

Mrs. Carter praised Barry's performance in office, saying he had "guided the city in the right direction" and become "an admired national figure" in the process.

Also endorsing Carter at yesterday's press conference were City Council members Polly Shackleton (D-Ward 3), Charlene Drew Jarvis (D-Ward 4), William R. Spaulding (D-Ward 5) and Wilhelmina J. Rolark (D-Ward 8).