Rep. Mike Capuano’s bid for Massachusetts Senate has been gaining momentum, and he has succeeded in narrowing the Democratic primary contest to a two-person race between himself and frontrunner Martha Coakley, a number of party strategists and Massachusetts political observers say. But with the Dec. 8 election just days away, Capuano has yet to prove he has the ground strength to overtake Coakley, who has the advantage of a statewide campaign infrastructure thanks to her successful campaign for state attorney general.

Despite the grass-roots efforts of Alan Khazei and the hefty spending of Stephen Pagliuca, they’re viewed as long shots in the special election triggered by Sen. Edward Kennedy’s (D) death in August.

“It’s clearly Coakley or Capuano,— said Scott Ferson, a Boston-based Democratic strategist.

And while the dynamics of the two-and-a-half-month race — which Coakley has led from the start — have not changed dramatically, Capuano is “making a good late charge, as they say in horse racing,— veteran Democratic media consultant Dan Payne said. He is “piling up a pretty good group of supporters for the final days of the campaign.—

In the past week, Capuano, who represents the Boston-area 8th district, has rolled out endorsements from such Democratic heavyweights as former Gov. Michael Dukakis and Rep. Ed Markey, the influential dean of the Massachusetts House delegation, as well from the Boston Herald newspaper.

The nod from Dukakis, the state’s longest-serving governor and 1988 Democratic nominee for president, is particularly significant, Payne said.

There are still “thousands of people in Massachusetts who consider themselves Dukakis followers,— he said.

To try to reach them — and perhaps generate a headline or two at the same time — Dukakis recorded robocalls on behalf of the Capuano campaign.

“I think the Capuano folks have been very smart about creating some news every day,— said Christopher Greeley, Managing Director at Nelson Mullins Public Strategies Group in Boston and former chief of staff to Sen. John Kerry (D-Mass.).

He and other strategists also lauded the forcefulness of Capuano’s recent television ads. In one spot, the six-term Congressman goes after former President George W. Bush and former Vice President Dick Cheney by name, saying “I was one of the few Members of Congress to strongly oppose their Patriot Act, their torture and their illegal wiretapping.—

Ferson called that commercial “extremely effective.—

“Not only does it hit one of our favorite bogeymen, but it very nicely paints a picture of who Mike Capuano is and how he will stand out and fight, which people in Massachusetts like,— said Ferson, a former Kennedy press secretary. “I think he’s hit that nicely in the past couple weeks.—

Still, the consensus is that Capuano has more work to do if he’s to close the gap with Coakley, who enjoyed a 40 percent to 21 percent lead over Capuano among likely Democratic primary voters in a Boston Globe/University of New Hampshire poll released Nov. 23.

The outcome of the primary is likely to come down to which candidate is able to get their voters to the polls on an oddly timed and potentially frosty Election Day. Both Capuano and Coakley have received their share of influential union endorsements, but Democratic observers said that in a statewide race in Massachusetts, turnout is mostly influenced by campaign organizations, not outside groups.

The Coakley campaign says it is not relying solely on its existing organization for the Senate election.

“Clearly, because Martha has run statewide before, there is an infrastructure there,— spokeswoman Alex Zaroulis said. “The get-out-the-vote effort for the Senate race is way above and beyond that.—

Coakley has launched a five-day statewide swing to rally voters in the final days of the campaign, and Zaroulis said the campaign has captains in every state Senate district who are recruiting thousands of volunteers for canvassing and phone banks.

The biggest focus, she said, is simply to remind voters that there is an election on Tuesday, though she hoped two televised Democratic debates would also help get voters’ attention.

Capuano has proven his organizational chops in the past, outworking a crowded field of Democrats in 1998 to win his current House seat, thanks to strong turnout in his home base of Somerville.

“The problem for Capuano, as it would be for any Member of the House, is their organization and their name recognition— reaches “only a fraction of the state’s population,— University of Massachusetts professor Frank Talty said.

Coakley, meanwhile, started with a good statewide organization, he said.

Jim Gomes, director of the Mosakowski Institute for Public Enterprise at Clark University and a former Kerry aide, said that when he looked at the campaigns’ unfolding ground games, he did not see any Coakley rivals “having the kind of advantage in the field to make more than a point or two difference,— which, if the polls are accurate, wouldn’t be enough to win the nomination.

Ferson and Payne, however, said Capuano could have a shot if disinterest, weather or other distractions keep a lot of Democrats away from the polls.

“If this is a low-turnout race and the combination of liberal activists and Congressional colleagues can make sure [Capuano] voters get out, that’s how Capuano can win,— Payne said.

Khazei and Pagliuca, meanwhile, have not given up hope, even if the resident experts say their odds are long.

The Khazei campaign is emphasizing its grass roots and new media outreach, which it says have helped engage a passionate group of supporters and will produce strong turn-out on election day.

Khazei, a co-founder of the national community service program City Year, has “more followers on [social networking Web sites] Facebook, on Twitter, on LinkedIn, and more YouTube videos than any of the other campaigns,— spokesman Dave Jacobson said.

Khazei also received a boost from the endorsements of the Boston Globe and Worcester Telegram and Gazette newspapers, representing the state’s largest and second-largest cities, as well as from national Democratic figure Gen. Wesley Clark, the former NATO supreme allied commander. The campaign is touting the Globe endorsement in a new television advertisement launched Wednesday.

Payne predicted the Globe endorsement “probably will move him from fourth to third.—

Pagliuca, meanwhile, has the resources to continue to spend freely, thanks to personal wealth that he has acquired as a managing director of Bain Capital. The Boston Celtics co-owner has already loaned his campaign $5.4 million and has far outspent the rest of the candidates, with $5.6 million in disbursements through Nov. 18, his latest fundraising report showed.

“I’ve gotten more robocalls from Pagliuca than I have from the other three [candidates] put together,— Gomes noted.

Ultimately, however, there are few game-changing opportunities left in the campaign.

“There’s only three endorsements that could possibly in the last week sway the race,— Ferson said, “and they’re all named Kennedy — Vicki, Joe and Ted Jr.— — the late Senator’s wife, nephew and son.

There are no indications that any of the three are inclined to jump in the race at this point.

The winner of next week’s primary is expected to have a significant edge over likely Republican nominee state Sen. Scott Brown in the Jan. 19 special general election.