The gathering on the steps of City Hall sounded like a cross between a prayer meeting and a pep rally.

Members of District Council 37, one of the city’s largest public employee unions, called out “preach” when their leaders took the microphone, then chanted the name of their favored candidate, Comptroller John Liu, after he accepted the union’s endorsement for mayor.

But the press conference, last Wednesday, was not just a demonstration of enthusiasm. It was also about making a promise of material support.

Comptroller John Liu, a Democratic candidate for mayor, accepts the endorsement of municipal workers’ union District Council 37 in late May. Photo: Beth Morrissey

“We look forward to putting DC 37’s mighty army of volunteers into the field to help elect New York City Comptroller John Liu as New York City’s next mayor,” said Lillian Roberts, the group’s executive director, backed by dozens of union members clad in green T-shirts.

Liu too recognized the power of the union’s ground troops. “I know that with this army of green, we are going to win this election,” he said.

In New York City, the support of a union can make or break a political campaign. But not all endorsements are created equal, and divining the true value of union backing in this fall’s hotly contested mayoral election is more speculation than science.

Two groups with identical numbers of members can differ widely in their capacity to mobilize their members. By reputation, DC 37 is notoriously disorganized, and its political operation pales in comparison to that of SEIU 1199, which claims 200,000 health care workers in the city and Long Island. Local 1199 has endorsed Liu rival Bill de Blasio in the mayoral race.

And while the city’s rank-and-file police group, the Patrolmen’s Benevolent Association, is similar in size to the local grocery store workers union, the cops’ endorsement is far more valuable among voters who care about a candidate’s stance on public safety.

The city is home to hundreds of unions: public and private, construction and service, skilled and nonskilled trades. Each has its own interest in the outcome of the mayoral race — and unlike in some past campaigns, the labor movement is fragmented, thus far having failed to coalesce behind one candidate.

The lack of consensus puts a premium on support from individual unions with robust political arms. And that backing will mean even more in a race where many candidates for mayor are likely to opt into the city’s public financing program, which sharply limits spending.

But even backing from the most powerful groups — like the United Federation of Teachers (UFT), which has yet to endorse a candidate — will be no guarantee of a win.

Observers cited the race for mayor in 2001, when the teachers’ union endorsed three candidates in the primary, runoff, and general elections, none of whom won their contests.

“When Mike Mulgrew says ‘We make mayors,’ well, bullshit,” said Doug Muzzio, a political analyst and professor at Baruch College’s School of Public Affairs, referring to the UFT’s president. “It’s simply not true.”

A UFT spokesman declined to address Muzzio’s statement and passed along an article in which Mulgrew said that the union’s clout was “for others” to assess.

Political strategists and union sources admit that it’s almost impossible to tell just how much of an impact labor groups will have on the election. But they highlighted four ways in which union support can help tip the scales. They can deliver votes from their own ranks; organize their members to turn their neighbors and friends out to the polls; lend their brand to a candidate’s campaign; and pony up hard cash for SuperPAC-style ad campaigns.

New Yorkers can be sure that unions will be flexing each of these political muscles this summer, using whatever resources are their disposal.

Votes

In a city of eight million people, unions with tens or even hundreds of thousands of members can give candidates a direct boost at the ballot box. But just because a union’s leadership decides to back a candidate doesn’t mean that its members will fall into line behind.

“The question is: How much can they actually deliver of their own vote?” said Muzzio. “I don’t think there’s good data.”

Larger unions offer greater potential, but numbers don’t tell the whole story.

The UFT, with its claimed 200,000 members, is a potent endorsement. On the other hand, two more of the city’s largest groups — SEIU 1199’s health care workers and SEIU 32BJ’s buildings employees — have plenty of bodies, but many of them can’t vote, since they’re not United States citizens. How many? Not even the unions themselves can say.

“We’ve been reasonably aggressive in registration, but we don’t know what percentage of our members are citizens,” said Norman Adler, a lobbyist and consultant who’s working with 32BJ.

The police and firefighters, by contrast, likely have more eligible voters. But many of them live in the suburbs, so they can’t cast city ballots either.

Turnout also will depend on the stake union members feel they have in the election. Municipal workers, for example, have a bigger incentive to vote because they’re choosing their boss for the next four years — one who will decide how generous their new contracts will be.

Private-sector workers can be motivated by good contracts and pay raises delivered by top union brass, which make their members more inclined to vote for candidates endorsed by their leaders, experts said.

That’s been the case for the city’s Hotel and Motel Trades Council, said Scott Levenson, a union consultant and founder of the Advance Group. Though the council only has 30,000 members, its endorsement “gives you bang for your buck,” Levenson said.

Union organizers said they are constantly trying to motivate volunteer work by highlighting potential financial returns to members. If a certain candidate appears more likely to promote real estate development, for example, that makes it much easier to get construction workers behind them.

“I do this all the time…draw a direct correlation between political action and job creation,” said Mike McGuire, the political director for the Mason Tenders District Council. “Your members, they have to see that.”

Organizing

The unions’ backing for their chosen candidates goes beyond votes. Many, especially larger groups, have employees specifically dedicated to strategizing and lining up volunteers to hand out fliers and make phone calls when election season rolls around.

“They can afford to have big staffs thinking of nothing else, all year long, other than how to organize around election time,” said Bill O’Reilly, a Republican political consultant.

The gold standard of these operations is SEIU 1199’s, organized by Kevin Finnegan, a political strategist who has worked with Harvey Milk and Hillary Clinton. At least half of 1199’s members contribute $5 to $10 monthly on top of their union dues to the group’s political action committee — money that will help pay for mailers and phone calls in the mayor’s and other races.

The union has endorsed Bill de Blasio for mayor, and will put its members to work rallying votes — mostly from their families, neighbors, and church groups, since the city’s campaign finance rules largely limit unions to communicating with their own members.

Other muscular organizers include the Mason Tenders District Council — which says it represents some 15,000 construction workers — the hotel workers, SEIU 32BJ, and the teachers union.

DC 37 plans to tap its members to drum up support for Liu. It will also rally retirees, of which it claims to have 50,000. According Audrey Iszard, executive vice president of the DC 37 retirees’ association, campaigning can be a family affair, which can involve even those who are too young to vote. “The children will give you a button and say, ‘You’ve got to vote, because I want my mom to have a job,’” she said.

Still, despite its large ranks, DC 37 is widely viewed as lacking the juice to mobilize members en masse. The group is a coalition of various smaller local unions, which “have a tendency to go off on their own” in defiance of leadership, Adler said.

“They’re a mess,” he added.

President Lillian Roberts did not address a question put to her at the Liu rally about the state of the union’s campaign organizing efforts. A spokesperson for DC 37 did not respond to inquiries.

Cachet

If you’re a candidate, a labor endorsement is typically good for at least a press conference.

But after those first-day headlines, some union brands have staying power that can be valuable for mayoral hopeful to tout as the campaign plays out.

Foremost among these are the law enforcement unions, including police and firefighters, whose endorsements candidates can use to bolster their credibility with city dwellers concerned about public safety.

“If that issue is important to them, then the endorsement of those unions signals something to those voters,” said Muzzio, the professor.

A coalition of 18 such groups voted last week to endorse Democrat Bill Thompson  — though some of the most influential, like the Patrolmen’s Benevolent Association, have yet to back a candidate.

In the past, the law enforcement mantle has meant a lot to candidates: in 1993, the PBA helped topple David Dinkins by endorsing his opponent in the Democratic primary, then supporting Rudolph Giuliani in the general election.

“The PBA was instrumental, rightly or wrongly, in labeling David Dinkins as weak on public safety,” said Levenson, the union consultant.

As crime has declined, those endorsements have come to mean less, but strategists say they’re still significant.

The teachers’ support can help, too. Levenson, whose firm has worked for the union, said that the UFT’s endorsement could be considered more influential than the New York Times’.

But others said that the UFT’s backing could be a mixed bag, given the fierce fights over education playing out through the mayoral race.

“Having the teachers helps in some quarters and hurts in others,” said Adler. “There’s a lot of people out there who are mad at the teachers.”

Sometimes the brand-name recognition comes not with the unions themselves but with their colorful and media-savvy bosses. Prime examples include the corrections officers’ Norman Seabrook, and the retail workers’ Stuart Appelbaum, who was already issuing press releases touting City Council Speaker Christine Quinn’s credentials nearly a year before her official announcement of her mayoral campaign.

Through a string of press releases and public statements, Appelbaum has pumped up Quinn’s liberal credentials, giving her cover on the compromise she brokered on paid sick leave legislation. He’s also gone on the attack to blast Quinn’s Democratic rivals, calling Bill de Blasio’s suggestion that she had watered down the legislation to please business allies “stupefying.”

Independent expenditures

Independent expenditures are the wild card in elections, an unpredictable element that could, under the right circumstances, sway key voters.

It remains to be seen if the flood of outside dollars that deluged state and federal elections last year will also descend on the New York City mayoral race. But one outside group, with substantial labor backing, is already spending money to influence local voters.

New York City is Not For Sale has launched a campaign against Quinn. So far the effort, which includes two television ads, fliers, and a website, has been largely funded by Communication Workers Local 1180, a union claiming to represent more than 8,000 members. After the ads launched, the union endorsed John Liu.

According to filings with the New York City Campaign Finance Board, the union contributed just over a half million dollars to the effort, while two political activists each contributed $200,000.

CWA Local 1180 also funded an attack ad campaign against Mayor Michael Bloomberg in the 2009 elections. That salvo was launched a few weeks before the election, while the effort against Quinn began much earlier — about four months before the primary.

“Our concern this time was that there was momentum building in the primary,” said Arthur Cheliotes, president of CWA Local 1180. “You can win the primary, and it’s done.”

After the TV ads aired, the Quinn campaign asked Time Warner cable to pull them from the air on the grounds that they made false claims.

But that’s not stopping Cheliotes. He said the group has received a new infusion of cash from supporters, and that it plans to spend a total of $1.5 million — an amount he contends is significant enough to influence the election.

“I think if you can strike a chord, if you understand what people are feeling, and [they] need to have it affirmed, then it doesn’t take much,” he said.

The decision to air television ads this early in the election cycle can also be cost effective.

“If you’re going to put money on an independent expenditure, the price point increases as you get closer to primary day, because you’re competing with more people on the airwaves,” said Levenson, whose Advance Group was hired by New York City is Not For Sale. “One of the reasons we launched our independent expenditure early is because we recognized that there was a bit of a vacuum on the political airwaves.”

But the timing of campaign ads is not an exact science, and opinions are divided about when in an election cycle outside funding can make the greatest impact.

According to SEIU 1199’s Finnegan, it would take about $1 million a week to significantly affect the mayoral race through television spots — a sum he says other unions are more likely to spend if they turn out to be united in their endorsements.

But just because they can make major investments in ad buys or sending mailers to non-members doesn’t mean more unions will do so this year.

Such moves could make an impact in the September primary — but according to Adler, the consultant with 32BJ, if unions are going to “slip into their swimsuit and jump into the pool,” they want to be sure that it’s worth the price. And that’s not clear yet, he added, given that many of the Democratic candidates have similar — and often vague — positions on key issues.

“A lot of the unions I talk to don’t know which way to go. Most feel none of these guys, if they get elected, will really harm them,” Adler said. “And that’s the threshold question.”