Honolulu Mayor Kirk Caldwell’s re-election bid received a key endorsement Thursday as potential challengers Ernie Martin and Peter Carlisle continued to take small steps toward their own campaigns this fall.

Caldwell’s endorsement by the 15,000-member Hawaii Construction Alliance is the first of the fledgling 2016 mayor’s race, which is only three months away.

The alliance consists of construction-related union organizations representing a combined 15,000 workers. The organizations include the Hawaii Regional Council of Carpenters, the Operative Plasterers’ and Cement Masons’ Union Local 630, the International Union of Bricklayers and Allied Craftworkers Local 1, the Laborers’ International Union of North America Local 368 and the Operating Engineers Local Union No. 3.

Tyler Dos Santos-Tam, Hawaii Construction Alliance executive director, applauded Caldwell for supporting key road, sewer and parks projects that are critical to the island and have provided jobs for alliance workers.

Martin, the City Council chairman, is increasingly looking like a candidate.

Last week he pulled nomination papers from the city clerk’s office, but he has not filed them.

Another possible sign of Martin’s plans is that for the first time, his campaign committee’s required fundraiser disclosure form, for a Waikiki gathering Thursday, identified him as running for mayor. Previously, his fundraiser disclosure forms said he was raising money for re-election to the Council, even though he would not be eligible to seek third term of his Council seat in 2018.

But Martin told the Honolulu Star-Advertiser on Thursday not to make too much of the “mayor” designation on his disclosure form. He still has not made up his mind whether he intends to run this year or in the future, he said.

Candidates for the 2016 election must file nomination papers by June 7.

Martin said he expects to make a decision “before the end of the month, definitely.” His campaign staff has researched the state “resign-to-run” law and does not feel he needs to resign from the Council until he formally files nomination papers, even if he makes statements declaring his candidacy for another office, he said.

One factor he will consider in making his decision is how much money he will have to battle Caldwell, he said. “The incumbent has a significant amount of revenues available at his disposal. I’m not even close to what he has. To run for a countywide election, it takes significant resources.”

In campaign financing documents filed Jan. 31, the Caldwell camp reported a $1.6 million war chest. Martin’s campaign had collected $492,510.

The Martin fundraiser Thursday night was his campaign’s third of 2016. Caldwell had his own fundraiser Thursday night, at Waialae Country Club. It is his campaign’s fifth event of the year.

Carlisle, Caldwell’s predecessor, told the Star-Advertiser on Wednesday he is leaning toward running for mayor and will make a decision soon. Carlisle, in his 2012 re-election bid, finished third behind former Gov. Ben Cayetano and Caldwell in the August runoff.

Carlisle’s last campaign spending report was filed at the end of June 2013 and showed a zero balance. The petercarlisle.com page is not active.

Former Lt. Gov. James “Duke” Aiona had also been considering a bid for Honolulu mayor. But he told the Star-Advertiser on Wednesday that he is now inclined not to run, but won’t make a final decision about his 2016 campaign year plans until later.

The first special election for the nonpartisan mayor’s race is held in conjunction with the Aug. 13 primary election. If the first-place finisher gets more than 50 percent of the votes cast, he or she is declared the winner. If the top vote-getter in that election fails to meet that threshold, the two top finishers go head to head in the second special election on Nov. 2, general election day.

Caldwell filed nomination papers last week. Five others have done so to date: Kurtleigh Baker, Lawrence Friedman, Timothy Garry, Angela Kaaihue and Joseph Wargo.