Two and a half years after the 1976 presidential election, the Federal Election Commisson has not issued its required audit of Jimmy Carter's campaign finances.

The commission. citing a confidentiality requirement regarding audits, will not say what stage the Carter audit is at now.

But two potentially relevant deadlines are approaching for the commission; the June deadline for claiming repayments of federal matching funds for the Carter primary campaign and the three-year statute of limitations after which violations of campaign finance laws cannot be prosecuted.

Beyond that, the commission is in the embarrassing position of being, in effect, lapped by Carter's 1980 campaign, which formally filed its papers with the FEC yesterday under the banner of the Carter-Mondale Presidential Committee.

The 1976 Carter primary committee, called the Committee for Jimmy Carter, and its successor for the general election called the Democratic Presidential Campaign Committee, received about $25 million in federal funds under the public financing law used for the first time in that campaign.

The audits of the candidates' finances are required largely to determine if the public money was spent properly and that the spending was documented properly.

The campaign audit for then President Ford was completed a year ago. Besides the Carter campaign, audits are still uncompleted for Democrats Fred Harris, Sargent Shriver and Morris Udall, all of whom dropped out before the nominating convention in 1976.

The Complicated aucit process revolves primarly around the documen tation of expenditures by the committee. The Carter campaign, sources said, had a particular problem gathering the documentation for items questioned by the commission -- sometimes items as small as $100 or $200.

The problem, sources said, did not stem from any lack of cooperation from Carter campaign officials but from imprecise recordkeeping in a far-flung operation that used scores of bank accounts in numerous states.

Richard Harden, a campaign finance official who is now an assistant to the president for administration, said he recalled that at one point the FEC was questioning the documentation for more than $80,000 in spending. He said that figure was eventually whittled down to about $4,000 or $5,000.

Others sources said that the most time-consuming chore for auditors involved the transfers of campaign money to various political committees around the country which conducted "get-out-the-vote" efforts. Spending in this area is especially sensitive because of the use of "walk-around money" by such committees, a loose and general unmonivored practice by which local political bosses dole out small amounts in cash to people who work around the polls on election day.

The FEC, according to sources, also spent time going over campaign payments to the Gerald Rafshoon Advertising Agency, which handled the Carter account. The payments were generally delayed for long periods during the primary campaign and, in effect, amounted to hundreds of thousands of dollars in credit.

Carter campain officials and others familiar with the FEC's work said that all of these matters have been cleared up and any serious questions resolved.

FEC Chairman Joan Aikens would not comment on this. She and other FEC officials said they were bound by provision of the election law which prohibits disclosure of information about audits.

"We expect it soon," said Aikens. "The audit is in house," she said, declining to elaborate.

"Policy questions had to be decided by the entire commission," she said when asked about the delay. "The committee [campaign committee] requested extensions of time to supply documentation. It's been a question of getting responses to imquries that have been made." she said.

The Ford audit was completed earlier, she said, because there was "a difference in the basic bokkeeping systems. Ford had a system that gave us documentation that was not readily available from others," including Carter.

Harden said the 1976 campaign committee was still win the items" with the commission.