Former president Carter, returning to the Washington political wars, said yesterday that President Reagan has made such fundamental blunders in economic policy that they can hardly be undone.

In his first press conference since he left the White House and in a brief appearance before the staff of the Democratic National Committee, the man Reagan defeated dismissed the new administration as "an aberration on the political scene."

He lit into Reagan for sponsoring "unfair and excessive" tax cuts, which, he said, will lead to "massive deficits" and "drastic" reductions in programs for the elderly and needy.

Carter told the DNC staff that the Democrats would profit in 1982 from Reagan's errors, and suggested in the session with reporters that the same may be true for 1984. "It is hard to understand what could be done to correct the serious errors that have been made . . . unless the decisions are undone," he said.

Appearing relaxed and in good humor, Carter dashed from the DNC to a luncheon with House Democratic leaders, to a press conference in a downtown hotel and then over to the White House for a meeting with Reagan.

Carter told reporters he would use the session to urge his successor to rethink some of his recent defense decisions and to seek Senate ratification of the SALT II strategic-arms treaty with the Soviet Union--both steps Reagan was almost certain to reject.

After the 35-minute meeting, Carter declined to discuss the substance of the talk, but said it was "a good, constructive and friendly meeting."

Despite the barrage of anti-Reagan rhetoric he laid down on his travels, Carter received a distinctly cool reception from Democratic leaders, because of his support--announced Monday and reiterated yesterday--for Reagan's proposal to sell AWACS radar-surveillance planes to Saudi Arabia.

Democratic National Chairman Charles T. Manatt, who went out of his way to say that it was Carter who suggested the visit to the DNC, rejected the former president's position on AWACS. "The president speaks for himself," Manatt said of Carter. "We're 100 percent committed to opposing the sale."

Later, at the Capitol, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said of Carter and the arms deal: "He didn't convince me four years ago. I guess he can't convince me now."

While these rebuffs served as a reminder of Carter's often-strained relations as president with party and congressional leaders, he was clearly enjoying his opportunity to re-emerge as a Democratic spokesman. Asked by a reporter if he might consider running for president again, Carter said, "I have no ambitions along that line at all." But he said he felt he had "not only a right but a duty to speak out," and that after his book on his presidency is completed next spring, "I will be much more active speaking out for my party."

Except for the AWACS deal, Carter found little to applaud in the work of his successor. He said the constant emphasis on building new arms and shipping them overseas, unaccompanied by diplomatic initiatives for a Middle East peace settlement or arms-control agreements with Russia, created a damaging perception that the United States is interested only in "the military angle of every question."

Carter reiterated the position he and former president Ford took jointly in their interview Saturday night aboard Air Force One, returning from the funeral of Egyptian President Anwar Sadat, in favor of U.S. talks with the Palestinian Liberation Organization.

Carter said the talks could come "simultaneously" with PLO recognition of Israel's permanence as a sovereign state. He said his suggestion was not prompted by any discussions with the Reagan administration. Rather, it rested on his belief that "there can be no peace in the Middle East without resolving the Palestinian question" and that there is "no prospect of any other group representing the Palestinians" than the PLO.