A bitter and close fight for the presidency of the Uniformed Firefighters Association ends Monday.

The campaign has been closely watched by the Bloomberg administration, whose relationship with the current leadership of the city’s major fire union has been characterized by personal attacks and public rifts over contract negotiations, response times, staffing, safety equipment, and even dress code at the St. Patrick’s Day parade.

And the outcome of the election between the incumbent, Stephen Cassidy, and a 27-year department veteran, Thomas DaParma, could prove crucial to the union’s negotiations with the city for a contract that is nearly three years overdue.

The city’s teachers, police, and fire unions are without contracts and are expected to make a big push against the mayor in his re-election bid. Firefighters Association ballots were mailed out May 26 and are due back by Monday, when the outcome is to be announced. The next three-year presidential term begins August 1.

Historically, the fire union, which has 8,800 members, has demonstrated a tendency to dump incumbents and vote for change. Several of their presidents have served more than one term, but rarely consecutively. This year, thousands of new union members, who joined the force after 2002, are voting for the first time. Historically, the Brooklyn candidate has had an advantage because that borough is where the most firefighters work, requiring the challenger to carry at least three of the other boroughs. Mr. Cassidy is from Brooklyn and Mr. DaParma is from Queens.

Although grassroots union campaigns used to be run out of collection boxes filled with $10 and $20 bills, contemporary candidates sometimes spend $100,000 or more on their campaigns, visiting many of the city’s 197 firehouses, sending direct mailings, producing and distributing fliers and videos, designing and maintaining Web sites. Recently, candidates have hired pollsters to test feelings about the issues and learn their strengths and weaknesses. Money is generated through fund-raising events and contributions – which are not regulated by any city, state, or federal agency, unless allegations of improper contributions are filed with the U.S. Department of Labor. One source close to the campaign suggested that the lack of oversight favors incumbents, who may solicit large contributions from labor lawyers and others seeking to extend their contracts with the union.

Mr. Cassidy earns more than $100,000 a year, about half from his firefighter’s salary and half as union president. Mr. DaParma recently retired to devote himself to his campaign.

The long-contentious relationship between the union and the Bloomberg administration, and the stalled contract negotiations, led some observers to wonder whether the mayor’s office would try to influence the election, in an effort to usher in new leadership. In theory, that intervention could come in the form of campaign contributions or more indirect means, such as suggesting to second-tier union officials that a change in leadership might produce more favorable contract terms.

Although conventional wisdom suggests that Mayor Bloomberg would prefer a change in union leadership, the administration’s moves seem to have had the opposite effect, of granting Mr. Cassidy free publicity. On May 26, the same day the election ballots were sent out, the fire commissioner, Nicholas Scoppetta, accused Mr. Cassidy and another fire union boss of encouraging elevated response times to gain leverage for their push to reopen firehouses and increase staffing. Mr. Cassidy’s return volley of recriminations aimed at Mr. Scoppetta appeared all over the city press.

“That was beneficial to the incumbents,” a former president of the union, Jimmy Boyle, told The New York Sun. He said Mr. Cassidy’s criticism of the commissioner “looks good to the membership.”

A longtime political consultant in the city, Hank Sheinkopf, said gambling with involvement in this election would be too costly for the mayor.

“I can recall mayors gloating when people get beat, but direct involvement is generally not productive or smart,” Mr. Sheinkopf said. “If you take on a union boss, you had better win. … If Cassidy wins, he is not going to look very kindly on the people who campaigned against him.”

Mr. Cassidy has portrayed his challenger as the administration’s stooge, a weaker representative of firefighters’ interests in the continuing battle with City Hall. In an endorsement on Mr. Cassidy’s campaign Web site, union vice president, James Slevin, wrote: “We must not grant Mike Bloomberg his wish by voting out Steve Cassidy. If Mike Bloomberg and Nick Scoppetta had a vote in this election it would be for DaParma.”

In a tactic that infuriated some firefighters and initiated a barrage of letters to a prominent civil service newspaper, The Chief-Leader, Mr. Cassidy distributed a campaign flier that suggested favoritism in the hiring of Mr. DaParma’s son, Christopher, as a city firefighter. Both Mr. Cassidy and Mr. DaParma’s Web sites identify the “lies” of the other candidate.

“There have been a lot of personal attacks in this campaign, and it doesn’t serve anyone but the city,” the president of the Long Island association of retired New York City firefighters, Ken Dolan, said. “Calling one another liars, it sounds like the Democrats versus the Republicans.”

A spokesman for the mayor’s re-election campaign, William Cunningham, denied any involvement.

“You inject yourself into union politics and you are playing with fire,” Mr. Cunningham said.

A spokesman for Mr. Scoppetta also said the commissioner had no involvement in the campaign and didn’t even had a preferred candidate.

Both Mr. Cassidy and Mr. DaParma declined to comment for this article, citing their constitution bylaws that prohibit using the press for campaigning.