The Reverend Alford Sharpton stunned the Democratic candidates running for mayor last week when he announced that for the first time in 20 years he would not endorse a candidate in the primary, because no one in the field had presented a clear vision for the city. Candidates should view Rev. Sharpton’s comments as a wake-up call, not a rebuke, analysts said.

“There is no question that the Democratic message is in trouble,” one political consultant, Scott Levenson, said.

Mr. Levenson, president of the Advance Group, said the four Democrats running for mayor “have failed to make an argument about what they will do better than Bloomberg.”

“Until they make that argument,” he said, “the swing voters won’t be coming their way.”

Another Democratic consultant, Hank Sheinkopf, said: “It was a wake-up call. Sharpton’s telling them to make something happen. The fact is the incumbent is in trouble and they aren’t taking advantage.”

Mayor Bloomberg’s support is fragile. About half the city’s voters like the job he is doing. Pollsters say that in a hypothetical matchup, the former Bronx borough president, Fernando Ferrer, still beats the mayor by five percentage points. With the city comptroller, William Thompson Jr., opting out of this race, Mr. Bloomberg’s campaign advisers said they might be able to capture a decisive share of the black vote.

And now Rev. Sharpton seems to be indicating that that is true.

Remarkably, the possibility that Rev. Sharpton might be correct, that a persuasive Democratic message isn’t resonating with voters, seems to have been lost on the four people vying for the nomination to oppose Mr. Bloomberg.

The Manhattan borough president, C. Virginia Fields, the only black among the major parties’ seven mayoral candidates, said Rev. Sharpton’s decision to withhold support was merely payback for no one in the mayor’s race supporting his 2004 presidential bid. Mr. Ferrer’s aides declined to comment about Rev. Sharpton’s snub on the record.

City Council Speaker Gifford Miller and Rep. Anthony Weiner of Queens hadn’t anticipated winning a Sharpton endorsement in the primary but clearly have not changed their tactics in response to his decision not to back Mr. Ferrer or Ms. Fields.

“Whatever else you say about Sharpton, he is a brilliant strategist,” a Baruch College professor of politics, Douglas Muzzio, said. “It hasn’t occurred to any of them that maybe Sharpton’s right, maybe none of them have developed a compelling message. Sharpton is saying exactly what loads of people are thinking: The Democratic field haven’t given people a reason to vote out the mayor. And they need to do that if they want to win.”

Rev. Sharpton’s support has been viewed as critical for Democratic candidates in the past. Mr. Ferrer, who is of Puerto Rican descent, has been courting prominent black politicians and community leaders to try to build a black-Latino coalition to beat Mr. Bloomberg, and Rev. Sharpton was part of that calculus. Then in March, Mr. Ferrer told a group of police sergeants that the fatal shooting by police officers of the unarmed African immigrant Amadou Diallo was not a crime and that there was pressure to “over-indict” the officers involved. The remarks, which Rev. Sharpton called “troubling,” gave him an excuse not to endorse Mr. Ferrer.

That said, the lack of a message, what the first President Bush called “the vision thing,” is evident when the Democratic field gathers for community forums around the city.

Last Thursday night, at Hunter College, for example, the four candidates were on stage sharing a long, cafeteria-style table for two hours. Not one of the candidates laid out an overarching vision for the city. Instead the four took aim at Mr. Bloombergs’ status as a billionaire – in one of the lighter moments of the evening, Mr. Weiner described himself as a “thousand-aire” – and talked, seemingly over the audience’s heads, about Section 8 vouchers, federal housing programs, and education.