Large number of undecideds raises stakes in tonight’s mayoral debate

After two heated but civil debates in the preliminary election, and one in the current campaign, this debate is crucial, political observers said, because a pivotal bloc of undecided voters who typically tune in late to local races may be watching, affording Flaherty one last shot to make a case for change. Thirty-eight percent of likely voters said in a Globe poll published yesterday that they are still trying to make up their minds, even though 52 percent said that if the election were held today, they would vote for Menino. Flaherty got 32 percent in the poll.

“This debate will be absolutely essential for the people who will make an important determination at this last debate,’’ said Raymond L. Flynn, a former mayor who is backing Flaherty. “This will be decisive.’’

Menino, a 16-year incumbent who enjoys a vast fund-raising advantage and has a seasoned political operation, merely needs to keep his temper in check and avoid major mistakes in the face of Flaherty’s attacks, supporters of both candidates said.

“The mayor just has to maintain an even keel,’’ because so many voters already know him and like him, said state Representative Elizabeth A. Malia, a Jamaica Plain Democrat who supports Menino. “Flaherty has to convince people and light a spark that says, ‘This is substantial and a major improvement.’ ’’

State Representative Brian P. Wallace, a Flaherty supporter, agreed with Malia that the stakes are much higher for Flaherty than for Menino.

“The mayor needs to do what he does and hold his ground,’’ said Wallace, a South Boston Democrat. “He’s not going to come out and wow anyone with his oratory. He’s not going to make any rash statements, and a draw for him is a win for him. The expectations are kind of low, so he can’t make any major gaffes, which would be a story in the paper the next day, which he doesn’t need.’’

The hourlong debate will begin at 7 p.m. before an audience at the John F. Kennedy Library and Museum in Dorchester and be broadcast live on NECN, WGBH-TV (Channel 2), and WBUR-FM (90.9), which are cosponsoring the debate with The Boston Globe.

The debate comes at a critical juncture, as the campaigns turn their attention to courting undecided voters and making their closing cases before Election Day in two weeks. The task is likely to be more formidable for Flaherty, who won 24 percent of the vote in the Sept. 22 preliminary election to Menino’s 50.5 percent.

At tonight’s debate, “you’ll have a lot more people tuning in,’’ said Representative Martin J. Walsh, a Dorchester Democrat who said he is unaligned in the race. “There’s an opportunity to make up a lot of ground here in the debate, or lose ground, depending on who wins.’’

Flaherty is pinning his hopes on a high turnout among young people, professionals, and minorities who did not vote in the preliminary election. Flaherty believes those voters will be more inclined to vote for him and his prospective deputy mayor, Councilor at Large Sam Yoon; in the Globe poll, young voters and newcomers to Boston were the most likely to say they are still trying to make up their minds, even though the poll showed Menino running strongly among all demographics.

“Yoon and Flaherty need to energize that whole new stream of voters,’’ Wallace said. “Flaherty has to make some inroads into areas that he didn’t do well in, in the minority community. And he’s got to get some ministers on his side. He has work cut out for him, but there are a lot of things he can do.’’

Menino, in an interview, downplayed the stakes of the debate.

“I just have to be me, just be out there and present my case,’’ he said last week. “I just got to go out and do my job. I talk about the future of Boston and not the past.’’

Flaherty, in a separate interview, said he was eager to spar with Menino again.

“What the debate does is remind people there’s a very important race for mayor of Boston, and the future of Boston is at stake,’’ he said.

Flaherty said he would continue to focus, as he has in past debates, on attacking Menino’s record on education, crime, and employment, which were the top three concerns cited by residents in the poll.

“I’m not sure what else people need to learn, to see that it’s time for new leadership in the city of Boston,’’ Flaherty said.

Menino sought to draw a contrast between his depiction of a city that is heading in the right direction, and Flaherty’s focus on lingering problems.

“This campaign is all about two cities: a city that keeps on moving forward, and a city that somehow other folks want to think is falling apart,’’ Menino said.

Yoon, meanwhile, dismissed the Globe poll and said the campaign had taken an internal poll that showed Flaherty trailing by a smaller margin. “We’re comfortable with our poll, which suggests that we’re 7 to 9 percentage points behind,’’ Yoon said yesterday in an interview on WRBB-FM.

The candidates are scheduled to attend one other debate, sponsored by MassVote, a nonprofit voting rights group, and moderated by Charles J. Ogletree, a Harvard Law School professor, on Oct. 27 at Faneuil Hall. There are no plans to televise that forum.

Michael Levenson can be reached at mlevenson@globe.com.