For the second time in six days, the proposed D.C. voting rights amendment was defeated by a single vote in Maryland's House of Delegates today, making it unlikely that the measure will survive its strong rural and conservative opposition this session.

The House will have another chance to approve the amendment when it receives an identical proposal passed by the Senate last week. But once-bullish supporters concede the momentum has turned against them and prospects of passing the measure are dim.

"In the kindest terms, I'd say the chances are bleak," said Del. Helen Koss, whose committee sent the amendment to the House floor by a vote of 18 to 2. Asked if her committee will approve the Senate bill, she replied, "We don't like to comment hara-kiri."

The amendment -- which would give the District of Columbia full congressional representation ratified by 38 state legislatures -- received 70 votes in the House today, one short of the constitutional majority required for passage. Sixty-eight delegates voted against the proposal.

As happened last week, sponsors of the amendment entered the session believing they had enough votes for approval. But they came up one vote short after discovering that two of their allies were absent and three delegates who supported the measure six days ago had changed their mind.

The strategy was bad," acknowledged Del. Walter Dean (D-Baltimore), one of the sponsors. "We didn't have a good count, which guys were wavering. Once we lost this the first time, we were put on the defensive. And we didn't work hard enough."

The problem for floor leaders was less one of winning allies as one of assembling all the measure's supporters on the House floor at one time. In the past six days, as alliances continually shifted, 79 different delegates voted for the measure at one time or another.

But sponsors could not assure attendance by all of their supporters at either of the two crucial votes. They also found today that they could neither depend on the delegates who supported the measure last week nor pick up enough new support to offset the effect of detections.

Del. Robin Ficker, a Montgomery County Republican, who voted for the amendment in committee and in the House last week, was one of three delegates who changes their minds today. Since last week, he said, he conducted a poll of his constituents and found they opposed the amendment by a 3-to-1 margin.

Ficker was one of a handful of Washington suburban delegates who voted against the measure, although many of his colleagues from Montgomery and Prince George's counties who supported the amendment confirmed his reading of public sentiments.

Some of them confided to their colleagues that they were torn between conflicting pressures -- opposition from constituents who fear D.C. representation in Congress would result in a commuter tax and support from organized labor and the Catholic archdiocese of Washington.

The way some of them resolved the conflict was to vote for the measure while privately asking friends from other sections of the state to vote against it, according to House sources. "It's called covering your behind," said one Montgomery delegate.

The public display of support by Washington's suburbanites and their pro-amendment allies in Baltimore was not strong enough to counter the assertive opposition of a coalition of conservative Republican and rural members.

"It's the view that the two senators (from the district) wouldn't have any conception of what rural sections need," said Del. Michael Sprague, a Democrat from Charles County. "There's no agriculture in D.C., no seafood industry in D.C."

The District's nonvoting congressional delegate, Walter Fauntroy, said today he was "extremely disappointed for the second time in a week" at the House's failure to pass the amendment, although he said he could "take comfort" that the amendment still has another chance.

Since the amendment was sent to the states for consideration last August, New Jersey, Ohio and Michigan have ratified it and seven other states, including Maryland, have rejected it in one way or another. If the amendment is to take effect, it must be ratified by legislatures of 38 states before August 1985.

The House vote today was an almost precise reflection of the split between urban and rural interests in Maryland.

The amendment attracted the support of about 90 percent of the delegates from Baltimore City and the suburban Washington counties of Prince George's and Montgomery, but could find only 10 votes from the rest of the state. Eight of those 10 came from the more urbanized -- and liberal -- sections of Baltimore County. Only two delegates from what could be called the rural areas -- Anne Baker of Howard County and Roy Dyson of Charles County -- supported the measure.

The vote also mirrored a division within the House leadership, which was so evenly split on the issue that it decided not to take a position. Many supporters of the amendment today criticized the leadership for staying out of the debate, saying that was one of the major factors in the measure's narrow defeat. "There's no doubt it would have passed if the leadership had gotten involved," said Robert Redding, chairman of the Prince George's delegation.

House Speaker Banjamin L. Cardin, who endorsed the voting rights amendment several months ago, said he urged the leadership not to endorse the measure because he was afraid such a move would alienate too many delegates.

With the leadership's unwillingness to push the amendment the task fell in the hands of two Prince George's delegates, Democrats Charles Blumenthal, the prime sponsor and Nathaniel Exum, a leader of the black caucus.

Blumenthal is considered a gadfly by many of his colleagues and has never been fully accepted by the House's inner circle of leaders. Today, he visibly irritated some delegates by referring to Raymond Beck, the Republican minority leader from Western Maryland, as "the delegate from Dundalk (a blue-collar town in Baltimore County), or someplace like that."

Exum denied the charge, but several members of the black caucus admitted that they are considering voting as a bloc against measures pushed by delegates who opposed the amendment.

Throughout the debate on the amendment, the black legislators have refrained from accusing opponents of being racially motivated. But Roy Dyson, an amendment supporter from rural Charles County, said he believed race was an unspoken factor in the vote.

"It was significant, but not articulated," said Dyson. "The fact that the city (Washington) has a black mayor, that it's an urban area -- that's probably frightening to some."