Almost since the inception of the special election to fill the late Sen. Edward Kennedy’s (D-Mass.) seat, state Attorney General Martha Coakley (D) has been considered the runaway frontrunner.

But with one week to go before Bay State voters head to the polls, Coakley is now in a tightening contest trying to fend off an unexpectedly competitive challenge from state Sen. Scott Brown (R).

The political narrative — a presumptive special election winner faces a stronger-than-anticipated challenge from an upstart Republican who seems to come out of nowhere — is a familiar one in Massachusetts.

That was the same story line in the weeks leading up to the 2007 special election to succeed Rep. Marty Meehan (D) in the 5th district, and now Coakley finds herself in a similar position.

The parallels between the two races underscore the unpredictable nature of special elections, even in dark blue states like Massachusetts, as well as the perils of complacency in frontrunner campaigns.

One strategist backing Coakley also cited the 2007 race, which now-Rep. Niki Tsongas (D) ultimately won by a closer-than-expected 6-point margin over retired Air Force officer Jim Ogonowski, to illustrate why the party should not hit the panic button.

“Look, these special elections are a complicated matter,— he said. “Turnout is such a question mark.—

However, he said, “We feel good that [Coakley is] going to win on Jan. 19.—

A number of other Democratic strategists in Washington, D.C., and Boston agreed that Coakley remains poised to win the race, though they do not dispute the fact that the race has tightened considerably in recent weeks.

“Democrats should be worried because the race has gotten this close. There’s no way to spin it,— said Scott Ferson, a Democratic political consultant based in Boston. But “will Martha Coakley lose? I find that very hard to believe,— said Ferson, a former press secretary to Kennedy.

The polling picture is fractured: The Boston Globe, in a poll released Sunday, had Coakley leading Brown by 15 points, and an internal Democratic poll circulated Monday shows her up 14 points. But a Rasmussen Reports poll released last week showed Brown trailing by just 9 points, and a Public Policy Polling survey released Saturday showed the race in a statistical dead heat.

However, Scott Rasmussen of Rasmussen Reports observed in an analysis of the polling Monday that all three independent surveys “show Coakley right around the 50 percent mark,— which, he said, would make the race “hers to lose.—

In general, Democratic strategists believe Coakley is neither as far ahead as the most favorable polls show her to be, nor is she in a dead heat with Brown, as the PPP survey purports.

“I believe we’re comfortably ahead,— said one Democratic party strategist, who spoke on background. But, he added, “In this climate and this environment, I wouldn’t take anything for granted.—

The uncertainty, driven by the unpredictable turnout and national political unrest, has generated hope among national Republicans, who are flocking to jump on the Brown bandwagon, as well as a scramble among Democrats, who hope to counter Republican energy with their own Democratic turnout operation.

The lower the turnout, the more it benefits Brown, as Mark Blumenthal of Pollster.com observed in an analysis Monday. He noted that among extremely interested voters, the race was even in the Globe and Rasmussen polls, but Coakley led comfortably when less committed voters were included in the tally.

The GOP enthusiasm for Brown has been swift and sudden. After campaigning in near obscurity since September, Brown’s campaign began to gain traction over the holidays, thanks in part to his pitch to become the 41st Senate vote against the Democrats’ health care reform legislation.

The American Future Fund is spending $400,000 on an ad attacking Coakley’s position on taxes, and Brown has received several endorsements from high-profile Republicans. A spokesman for Our Country Deserves Better said it plans to begin airing ads in the days leading up to the election.

On Monday, Brown raised more than $700,000 through a “money bomb— campaign online.

The national party, however, has yet to report any independent expenditure spending on Brown’s behalf, which would be a sure sign that the party thinks it has a chance to win the seat.

Democrats, meanwhile, blame the national climate, as well as Coakley’s low profile on the campaign trail of late, for Brown’s recent surge in momentum. But they believe the dynamics will shift in the race’s final week.

“The guys’ been on TV a lot,— the strategist supporting Coakley said. “He’s been communicating free and clear here for a while.—

“Martha just started her TV and its heavy, she’s got a lot of points behind it,— he added. Indeed, Brown is now airing his third TV ad of the general election, while Coakley is only on her first.

Ferson said Coakley needs to be more aggressive down the stretch. “You have to actually give people a reason to vote for you,— he said, likening her behavior to the Tsongas campaign in the 2007 special election. “You got to close the deal, and she has not closed the deal.—