James R. Schlesinger Jr., the man who became a public symbol of President Carter's foundering energy policy, resigned yesterday as White House aids voiced hopes that his departure would buoy the president's political furtunes.

Charles W. Duncal Jr., deputy secretary of defense, is Carter's choice to succeed Schlesinger.

Although Schlesinger said he will not leave the Cabinet until October, his departure as energy secretary is expected to bring to an end the chorus of criticism that had trailed the former Nixon and Ford administration Cabinet officer since he joined Carter in 1977.

As Schlesinger's departure was being announced the final drafts of the president's most recent Schlesinger-shaped energy plan were being readied for Congress.

Asked why accept another administration post? "i'm not planning another government job," he answered.

Were reported sharp policy differences with Stuart Eizenstat, White House domestic policy adviser, the key to Schlesinger's departure? "these reports tended to be exaggerated," Schlesinger replied, conceding that he was in favor of accelerating oil decontrol, a move Eizenstat is reported to have told Carter would jeopardize the president's political standing.

Then came the question that was on everyone's mind at yesterday's Energy Department news conference.

Asked why he was leaving the Cabinet, Schlesinger told reporters yesterday, "I did not believe I would be a asset to President Carter."

According to Schlesinger's congressional critics, his opponents in labor, environmental and consumer groups, and even on Carter's White House staff, Schlesinger's summation was the most poignant understatement of the two-day White House shake-up.

During the last 2 1/2 years, the often-embattled secretary was as much a part of debate as were the issues. Critics accused him of manipulation data, failing to deliver on promises and abondoning friends.

And, in one of the most extrordinary examples of suspicion in the administration's dealings with Congress, Sen. James A. McClure (R-Idoho) demanded that Schlesinger detail in writing a deal they had struck in exchange for McClure's vote on the gas bill last year.

Through it all, however, Schlesinger enjoyed Carter's continued support, political backing and a level of access not accorded most other Cabinet members.

In many respects, relations between Carter and Schlesinger were a Washington paradox. Carter, the Democratic populist who hedged on the use of nuclear power and was a proponent of stiff environmental standards and was an occasional critic of the oil industry's power, seemed the opposite of Schlesinger and his views.

Equally confusing for Carter's supporters was why the president chose a Republican who had served Nixon and Ford at the old Atomic Energy Commission, CIA and Defense Department to be the first secretary of energy.

As Carter's energy policy evolved over the last 2 years, however, it became more and more distant from his campaign pledges, and closer to Schlesinger's avowed preferences for ending price controls, eliminating environmental constrainst, exploiting energy resources and promoting nuclear power.

Underscoring these in his resignation letter, Schlesinger wrote: "The 30-year war over natural gas pricing has ended. The controls on oil prices --with their crippling effects -- are being phased out. "the nuclear option has been preserved -- and the age of renewables has been initiated."

Still another of Schlesinger's accomplishments, in the view of some, is that, despite Carter's occasional diatribes against the oil industry, his energy secretary developed a close working relationship with the major oil companies.

Asked yesterday whether his term at DOE was marred by any failures, Schlesinger answered, "1'd like to borrow a statement from former president Eisenhower: "Give me week, and I might be able to provide you with a list.'" The quote repeated Eisenhower's answer to query about what then-vice president Nixon was doing.

One legacy of Schlesinger DOE years is that the new department continues to be one of the least well-organized, most chaos-ridden government agencies.

A year ago Eizenstat said of Schlesinger: "Unlike any secretary in the Cabinet, he has been organizing the department while trying to get through comprehensive legislation. He has done as well as anyone could."

Since then, Schlesinger's reputation as a brilliant manager has run out. And even Eizenstat has said that the DOE continues to be one of the most poorly run government agencies.

Typical of the problems Schlesinger's successor will inherit is the mulitbillion-dollar strategic petroleum reserve program, now more than 1 1/2 years behind schedule and embroiled in controversy.

Duncan also will have to grapple with less momentous snarls, not the least of which is DOE's record for losing congressional mail. Schlesinger leaves behind a detailed energy plan of which Carter's reelection hopes are pinned.