An expected final count in the District of Columbia's Democratic mayoral primary race was abruptly postponed yesterday when it was discovered that election workers skipped a crucial step designed to detect possible double voting by some 2,000 challenged voters.

The D.C. Board of Elections and Ethics rescheduled the count to start at noon today in the District Building. Officials said they hoped the postponement would allow time for workers to complete the tedious paper work task of checking for double voters. If all goes smoothly; they said, preliminary but definitive vote totals in the razor-close mayoral contest should be available by tonight.

Results immediately after the Sept. 12 primary showed City Council member Marion Barry with a 1,188-vote lead over Council Chairman Sterling Tucker. Incumbent Mayor Walter E. Washington was 1,603 votes behind Tucker.

With about 3,000 absentee and challenged ballots - a greater number than the vote spread among the three candidates - still to be counted, a winner technically cannot be named yet. Today's scheduled count conceivably could swing the vote against Barry, but most election onservers and the candidates themselves acknowledge that the pattern of absentee and challenged ballots rarely varies greatly from the regular vote.

The failure of workers to check challenged voters for possible double voting was noticed by Phil Ogilvie, a Barry campaign worker who has been at the elections board every day since the primary.

A set of "Internal Guidelines and Procedures" drafted by board general counsel Winfred Mundle and distributed to employes describes in detail how workers are to check for double voting by challenged voters.

Voters, for example, who are registered in one precinct but have recently moved to another, may vote in their new precinct with challenged ballots. Their ballots are sealed in "challenge" envelopes with the name, address, old precinct number and other data on the face of the envelopes.

During the week since the primary, election workers were assigned to verify that such challenged voters were in fact duly registered. According to the written guidelines, they also were to check whether the voters may have additionally voted in their old precincts. This was to be determined by seeing if they had signed voter cards filed in their old precinct.

It was this procedure that was disregarded or overlooked. Workers began making the checks yesterday only after Ogilvie's complaint.

Informed of the oversight, Shari Kharasch, head of the three-member elections board, and board member James L. Denson ordered yesterday's scheduled final count postponed until today.

"If we can't train our people to work professionally," said Denson, "we're going to be in trouble."

It is not clear how the oversight occurred, and no one at the board would fix the blame on anybody.

Yesterday's postponement of the final count also came amid growing complaints by representatives of the candidates that security arrangements for the uncounted ballots have been lax during the past week.

The ballots and various voter verification records are supposed to be kept in a District Building vault under armed guard when not in use. Campaign workers claim, however, that some records were left unattended in the District Building ground floor hallway.

Election board officials acknowledged that voter card boxes from at least two precincts were lost temporarily. They contended there was no evidence of ballot tampering in any of the instances.

There is an air of quiet confusion mixed with fatigue in the elections office. Many of the 31 employes have worked for 15 to 20 consecutive days. Deputy elections administrator Delores Woods became ill at work yesterday and went home.

With the District's liberal rules on permitting challenged ballots, the board was flooded with about 4,675 challenges, plus 1,034 absentee ballots, after the Sept. 12 primary. Election laws specify that challenged and absentee ballots are to be counted one week after the election, to permit time for processing and examining the ballots. That one-week deadline was yesterday.

Of the approximately 4,673 have been rejected and will not be counted, mostly because the voters were unregistered or were independents and thus barred from voting in party primaries. The remaining 2,000 challenged ballots have been accepted and will be counted today along with the 1,034 absentee ballots.