Before the tally of the Ward 8 straw poll was announced, D.C. Mayor Vincent C. Gray answers a question at Turner Elementary School on Saturday. Gray and seven other mayoral candidates in the Democratic primary participated in a rowdy forum marked by heckling from the audience and verbal jabs from the candidates. (Marvin Joseph/The Washington Post)

Incumbent Vincent C. Gray failed to win a key mayoral endorsement Saturday from Democrats in the District’s southern corner, falling short of expectations in an early test of campaign organization and support among party activists.

Voters in the straw poll of 307 registered Democrats in Ward 8 preferred D.C. Council member Muriel Bowser (Ward 4), who became the first candidate to enter the race for mayor in March and whose campaign brought in dozens of backers to Saturday’s forum, mainly senior citizens.

Neither candidate received enough votes to win an official endorsement from the ward’s Democratic Party organization, which hosted the poll. But Bowser won a clear plurality, with 41 percent to Gray’s 31 percent.

Ward 8 includes Anacostia, Congress Heights and other neighborhoods that rank among the city’s least affluent. The area provided a base for Gray during his 2010 run, giving him 82 percent of the vote in the Democratic primary over then-incumbent Adrian M. Fenty. The 8,500-vote differential there provided nearly two-thirds of Gray’s victory margin.

Gray’s backers acknowledged Bowser had mounted a stronger effort to bring in supporters but said that was to be expected, given that the mayor entered the race only six weeks ago. “We didn’t move enough people to the straw poll today,” Gray campaign manager Chuck Thies said. “It is as simple as that.”

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Indications are that Gray retains strong support in the ward relative to the other candidates. A Washington Post poll released last week showed Gray with 34 percent support in the area east of the Anacostia River, which includes Ward 8. Bowser had 10 percent support there among registered Democrats in the survey, which carries a margin of error of five percentage points.

But the poll and Saturday’s results indicate that Gray may not be able to rely on the ward for the huge margins he racked up four years ago.

Before the tally was announced, Bowser, Gray and six other candidates participated in a rowdy forum marked by heckling from the audience and verbal jabs from the candidates, directed mainly at Gray.

Bowser said she would appoint a deputy mayor to focus solely on east-of-the-river neighborhoods, saying the Gray administration had not done enough to coordinate education and economic development there. Gray said the city needed a mayor, not a deputy mayor, focused on those issues. “And, oh, by the way, I live east of the river,” added Gray, a resident of Ward 7’s Hillcrest neighborhood.

One topic that was not broached during the forum was the federal investigation into Gray’s 2010 campaign, which according to the Post poll will be a factor in the decisions of three-fourths of city voters.

A few in the crowd thought differently. Vernon Humbles, 56, repeatedly shouted, “You’re a crook,” as Gray took the microphone. Humbles, a Greenway resident who works as a cab driver, said he had worked on Gray’s last campaign and was dismayed with revelations of a secret “shadow campaign” and felt that his devotion and that of other campaign workers had been disrespected by Gray.

“I saw hardworking people not get credit for their effort,” he said. “You have to disrespect him to his face, because he’ll disrespect you to your face.”

After the straw poll was completed, Bowser returned to the auditorium and looked at the tally posted on the wall. “The residents of Ward 8 have spoken,” she said. “They want a new mayor.”

Anthony Lorenzo Green, an Advisory Neighborhood Commission member in the ward, said he was not surprised by the tally. A supporter of Bowser’s, he said he has seen her “too many times to count” in the ward in recent months.

Council member Jack Evans (Ward 2), who had 15 percent support among east-of-the-river voters in the Post poll, received only 10 votes Saturday, finishing behind colleagues Vincent B. Orange (At Large; 40 votes) and Tommy Wells (Ward 6; 13 votes), as well as restaurateur Andy Shallal (13 votes). Reta Jo Lewis, a former federal official, received two votes; entrepreneur Carlos Allen received none.

Aaron C. Davis contributed to this report.