On March 21, the New York City branches of the Communications Workers of America held one-hour interviews in the union’s Pine Street office at Lower Manhattan to meet with Mayor Bloomberg and each of the four Democrats running for mayor.

Support from the union in the mayoral election would be valuable for the political troops it would deliver, but arguably even more significant is the influence that labor unions and other left-leaning organizations could have on the endorsement of the Working Families Party.

That endorsement, political analysts said, will translate into votes in the Democratic primary and, even more so, in November’s general election.

Though the Working Families Party has only 9,442 enrolled voters in the five boroughs, its clout at the polls is far greater.

The party’s executive director, Daniel Cantor, said he expects the seven-year-old party to pull many more votes in November’s mayoral election than the 32,551 it won for Mark Green, the Democrat who lost to Mr. Bloomberg in November 2001 by roughly 35,500 votes.

“We’re important as a ‘signaler’ in the primary and we’re important as a vote-getter in the general,” Mr. Cantor said during a phone interview last week.

Mr. Cantor predicted that at least 100,000 voters in November’s mayoral election would cast ballots on Row E, his party’s line, for the candidate Working Families endorses. If that turns out to be true, the party could outpoll all other minor parties in the city.

That prospect is not lost on the Democratic candidates or on Mr. Bloomberg, who made news in January 2004 when he met with Mr. Cantor and four other high-ranking members of the party at an Upper East Side restaurant.

Given the party’s track record, its endorsement is all but guaranteed to go to a Democrat, and many political observers expect it to go to the Democratic front-runner, Fernando Ferrer, a former borough president of the Bronx. The other candidates are the borough president of Manhattan, C. Virginia Fields; the speaker of the City Council, Gifford Miller, and Rep. Anthony Weiner, whose House district includes parts of Queens and Brooklyn.

The party’s endorsement timeline is flexible. It will probably make no decision until June and could choose to wait until the general election to make an endorsement, Mr. Cantor said. In the 2001 election, the party announced its endorsement of Mr. Green in August, the month before the primary. Party insiders said they expect a primary endorsement this time, too. Mr. Cantor said it was unclear which candidate the party would back and when it would make its decision.

The official process starts with a detailed questionnaire, which will be available to candidates in mid-April. The party will also schedule a forum style interview in May, in which members from the party’s various clubs and chapters can question candidates on everything from the city’s trash transfer system to education policy. From there, the party’s chapters and affiliates go home and decide whom to recommend to the party committee that ultimately votes on the endorsement.

“Not to downplay my own role too much, but I have very little power in this … my job is to be the honest referee here and make sure the process goes smoothly,” Mr. Cantor said.

In the four years since the last mayoral election, the Working Families Party has played substantial roles in dozens of races. In 2003, it raked in 15% of all the votes cast in City Council elections. A Brooklyn council member, David Yassky, for example, won 3,759 of his votes, or 31% of his total, on the Working Families line.

Moreover, that year, Council Member Letitia James – who represents Bedford Stuyvesant, Crown Heights, and several other Brooklyn neighborhoods – won her seat running only on the Working Families line. She took 76% of the vote against the Democratic nominee. The race was a major victory for the party in that it was the first election in the state in which a Working Families candidate won without the backing of a major party, and the first time since 1973 in which a candidate was elected to the council running solely on a third-party line.

In 1998, the party received 1% of votes on its ballot line. That number increased fourfold for 2003, when the Working Families Party estimates that its candidates picked up 112,000 votes.

“Candidates for mayor understand the value and importance of third-party support,” the executive director of the nonprofit Citizens Union, Richard Dadey, said. “It’s arguable that Rudy Giuliani would not have been mayor had it not been for the backing of the Liberal Party in 1993.”

The Working Families Party, founded in 1998 by a coalition of unions and labor activists, has grown in popularity with working-class voters. As a result, the labor community has a strong voice in making endorsement recommendations.

The legislative and political director of the Communications Workers District 1, Bob Master, who is also the co-chairman of the Working Families Party, said his union decided last week after its five mayoral interviews to hold off on its decision. Once the union decides whom to back, it will recommend that Working Families back the same candidate.

In 2001, like many other unions in the city, the Communications Workers endorsed the then-city comptroller, Alan Hevesi, in the Democratic mayoral primary and Mr. Ferrer in the primary runoff. Neither candidate won.

Mr. Master said that while union endorsements alone are important, many of them have added value because they influence the Working Families Party. “It’s an extra bit of juice,” he said.

A Democratic political consultant, Scott Levenson, said the Working Families endorsement is an important symbol of labor support and could even help prevent the union workers who support the mayor’s Jets stadium plan from coming out in full force for Mr. Bloomberg.

Working Families is not the only third party that will have carry significant weight in the election.

The Independence Party, which endorsed Mr. Bloomberg in 2001 and accounted for 59,000 of his 744,757 votes, is expected to throw its support behind the mayor again this year. And the Conservative Party is being wooed by Mr. Bloomberg’s two Republican challengers: a former City Council minority leader, Thomas Ognibene, and an investment banker, Steven Shaw.

The chairman of the New York State Liberal Party, Henry Stern, said no decisions have been made about endorsements. The party lost its “automatic” ballot status in the 2002 gubernatorial race but still could collect the petitions needed to get a voting line. Mr. Stern said he expected that there “would be a way” to vote for the candidate the Liberal Party chooses to endorse.

In 2001, the Liberal Party endorsed Mr. Hevesi, but he lost the primary and then backed the nominee. His name was on the Liberal line in the general election, but he won only 8,027 votes.

This time, Mr. Stern said, the party is looking for someone who will commit to running in the general election. The third parties do not hold primaries of their own.