Special elections are a fickle breed.

Their turnout is low, the partisanship of voters is high, and those running have to campaign without the energy boost of other candidates vying for office all around them.

In Massachusetts this June, that volatility could make for a tighter-than-expected contest in the race for John Kerry’s former Senate seat, according to a new analysis by the nonpartisan Cook Political Report.

The Cook report announced Thursday that it was shifting its prediction for the race from “leaning Democrat” to “toss up,” although the group acknowledged it still had its “thumb on the scale” for Democrat Edward Markey, an 18-term congressman who has consistently polled ahead of his Republican rival, businessman Gabriel Gomez.

“In truth, we have had a difficult time accepting the idea that this race might get close,” wrote Cook’s Jennifer Duffy. “At the same time, Democrats nominated a long-serving member of Congress at a time when Congress is an almost universally unpopular institution. It doesn’t help that Markey has not had a competitive race in decades.”

As the report notes, there’s still a lot going against Mr. Gomez in this race, right down to the basics: He’s a Republican paddling through a deep sea of blue. Registered Democrats outflank their GOP counterparts in Massachusetts by a margin of 3 to 1.

He also has less money, less clout, and less of an organizing base to kick-start get-out-the-vote efforts than does Mr. Markey, who brought first lady Michelle Obama to Boston earlier this week for a swanky lunch that raised more than $700,000 for his campaign.

Cajoling would-be voters to the polls will be key in this race, says Marc Landy, a political scientist at Boston College, because in a special election, “it’s all about turnout.”

Specifically, more than half of Massachusetts voters are registered as independents, and Gomez will need to get as many of them as possible to the polls June 25.

Recent history suggests that’s possible. In 2010, Republican Scott Brown won a similar special election on the strength of his support from independents, who voted for him 2 to 1 over Democratic Martha Coakley, according to a postelection poll by The Washington Post.

In a Public Policy Polling survey released in mid-May, Gomez had 56 percent of the independent vote – up from 47 percent at the beginning of the month.

But polls for special elections are notoriously hard to read, the Cook analysis points out, since it’s hard for pollsters to accurately predict who will vote. That’s led to a wild range of margins in this campaign so far, with polls showing Markey leading by anything from three to 17 points.

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Although Markey remains solidly ahead, this election is about more than who wins next month, Mr. Landy says.

“If Gomez were even to come close to Markey, that would indicate some real displeasure with the [Obama] administration in Massachusetts,” he says.