Because the election will be held on September 11, when there are no other state or federal elections to draw voters, most prognosticators expect turnout to be low, perhaps just 20 percent. The only thing that could change that is a huge influx of money, especially from national advocacy groups. In addition, Secretary of State William Galvin points to another factor that could lower turnout: the number of Democrats and unenrolled voters who are now registered as Republicans after voting for John McCain in the presidential primary. Galvin says he isn’t sure exactly how many voters switched parties, but he puts the number in the “thousands.” (Voters registered as Republican who want to vote in September’s Democratic primary have until August 22 to change their registration back to Democratic or independent affiliation.)

All this means that the candidate with the most committed base will have a distinct advantage. Just ask Mike Capuano, who beat nine other candidates in the race for the Eighth Congressional District after incumbent Joe Kennedy announced his retirement in 1998. Just 85,000 people showed up on Election Day, but Capuano, then the mayor of Somerville, was the only candidate with a solid base from which to draw; he won 23 percent of the vote, with onetime front-runner Ray Flynn placing a solid second.

Most observers agree that Stephen Lynch of South Boston will benefit similarly in the race to win the Ninth Congressional District. Lynch’s opponents hope that the string of negative stories about his financial problems, defaulted student loan, and tax lien will prevent him from growing beyond his base and, in the end, turn him into the Ray Flynn of the Ninth campaign. But the June 29 issue of the Rothenberg Political Report, an independent political newsletter, predicts that “low voter turnout would benefit the Southie Lynch,” and that turnout could be “as low as 15 percent.” The report goes on to observe, “South Boston voters have traditionally been among the most reliable in the district, and Lynch’s advantage with pro-life voters should also be an asset in a low turnout race.”

Nonetheless, even Stuart Rothenberg, the editor of the report, acknowledges that money could change the campaign considerably. “It’s all a function of how much the candidates spend,” he told the Phoenix, “how much comes in from national groups.” The other two potentially strong contenders in the Ninth, Brian Joyce and Cheryl Jacques (who hasn’t yet decided whether she’ll run), are counting on funds they can raise from progressive interest groups focusing on gay rights and abortion. Not to mention the television ads these groups could run — and pay for — to goose turnout.

Then there’s the role played by organized labor. To date, Lynch has picked up endorsements from almost 40 local unions, mostly in the trades — ironworkers, carpenters, bricklayers, and so on. But he’s not the only one to get union backing. Last weekend, Joyce announced that he would receive support from four unions: the International Brotherhood of Police Officers, the International Association of EMTs and Paramedics, the National Association of Government Employees, and the International Brotherhood of Correctional Officers. He also recently tapped political consultant Henry Sheinkopf, who has extensive experience working with labor unions. Joyce is scheduled to travel to Washington on Wednesday to meet with national unions and other traditionally liberal interest groups. Pacheco, meanwhile, is popular with labor but has yet to announce union endorsements.

The state AFL-CIO will not endorse any candidate until August 9, and the decision will require a two-thirds vote of the group’s executive committee. But union involvement is that much more critical when turnout is low, as AFL-CIO political director Richard Rogers expects it to be. Unions provide campaign volunteers and often play a decisive role in getting voters to the polls. Witness last November’s election in the Midwest, particularly in Michigan. Pundits predicted that President George W. Bush, with help from his GOP ally Governor John Engler, would win the state. But auto workers had Election Day off, and the unions launched a massive get-out-the-vote effort (in which new Joyce team member Sheinkopf played a part). Al Gore won Michigan.