Saying he would be a city councillor committed to "big ideas," John Connolly officially announced his candidacy for council at-large in Dorchester on Tuesday night at Florian Hall. It was the fifth such neighborhood-specific campaign kick-off that he has hosted across Boston to date.

Connolly has been considering another run at for at-large council since almost immediately after losing his first campaign for that body in 2005. Two years later, this year's race will be different in many ways: gone from the field are the son and daughter of two former Boston mayors (Ed Flynn and Patricia White) who were on the ballot that year. All four incumbent at-large councillors are running for re-election, unlike 2005, when Maura Hennigan left her seat to challenge Mayor Thomas Menino; and there is no mayoral election to pique voter interest.

"John's biggest opponent is low voter turnout," said Matt O'Malley, who finished sixth in the general election in 2005 and now works for Suffolk County Sheriff Andrea Cabral. O'Malley endorsed Connolly at Florian Hall on Tuesday night, describing him as an old friend and the candidate best suited to serve on the council.

As he addressed the crowd, Connolly alluded to the mutually beneficial optimism driving O'Malley's endorsement. O'Malley and Connolly both have strong, overlapping bases in Roslindale and West Roxbury, making for a natural alliance if they chose to run alongside rather than against each other.

"I know with the O'Malley team and the Connolly teams merged that now, and perhaps in future elections &endash; not for me, but for Matt &endash; I know we would both serve the city well."

O'Malley has indicated that he would strongly consider running for a council seat again in the future, but whether that would mean a third bid for an at-large seat or a run at the seat in his sixth district is unclear. That seat is currently held by six-year incumbent John Tobin, who is widely believed to be considering a run for mayor in 2009. If he vacates his seat to do so, O'Malley would be a strong candidate for the open seat.

While the period to challenge a candidate's authorized signatures is still open, it appears that all nine candidates who have filed papers with the election department will appear on the ballot for a preliminary at-large election on September 25. The field will be narrowed in that election to eight candidates who will vie for four spots on November 6.

Connolly is the most familiar name among the five challengers this year, but the unpredictable maneuvering that occurs between the preliminary and final election caused him trouble in 2005. After finishing third in the preliminary, he dropped to fifth in the general election, just short of a council berth.

"I'm the one that made mistakes between the preliminary and the final. You did the work to get us so close to victory," said Connolly to supporters on Tuesday night. One theory on Connolly's shift from a safe third to just out of the money is that supporters of incumbent Stephen Murphy and then-challenger Sam Yoon rallied behind their candidates by voting for them alone in the final.

"Too many people in Dorchester were throwing around a bullet vote in the final round last time," said Connolly, referring to a practice of voting for just one candidate instead of the four. "I want one of your votes."

State Rep. Martin Walsh, a strong Connolly supporter and the first elected official to endorse him in 2005, said that Connolly's experience would be a benefit in his second race.

"I had the pleasure to lose with John, and even though we were all upset, we learned something, and the only thing we can do now is win," Walsh told Connolly supporters at Florian.

Connolly has identified affordability, public education, and public safety among his top campaign issues this year.

In the two years since his last race, said Connolly, the importance of public safety has increased most dramatically.

"Dorchester is too great to be stigmatized by problems that must be solved by the whole city, not one neighborhood alone," he said.

Thelma Burns, a Dorchester resident who grew close to Connolly while both served on the board of Action for Boston Community Development, said she was impressed with Connolly's leadership skills and attracted to his personality, and most importantly, to his stance on education.

"I'm a supporter of neighborhood schools," said Burns.

An endorsement of neighborhood schools might be one of Connolly's more polarizing campaign positions, in a city sharply divided on a way to improve the public school system. It is also the common ground between Connolly and some of his strongest Dorchester supporters, like Pope's Hill Neighborhood Association leader Phil Carver.

"John and I are really kindred spirits in our ideas about what's good in this city, and what has the potential for change," said Carver. "A huge component of that, for me, is neighborhood schools." Related Story Two Dorchester men seek citywide seats