As Harvard’s battle to build a new government center reaches feverish pitch, the fate of its most important building project hangs in the balance.

Tonight the Cambridge City Council will meet to consider Harvard’s proposal to tunnel under a city street and connect the two main buildings of its planned Center for Government and International Studies (CGIS).

As recently as this winter Harvard officials had maintained that construction work on the new center would be underway by now. But they now concede they don’t have the votes they need on the council to clear the final hurdle—permission to dig the tunnel.

The five-year-long struggle will continue long after tonight’s meeting ends.

An all-out effort in recent months by the University to win permission for the tunnel has intensified—all geared toward tonight’s meeting of the city council, the first time Harvard will publicly present its case to the body that ultimately will decide the tunnel’s fate.

Tonight some of the University’s top administrators will head to City Hall, where Cambridge residents have promised they will turn out in full force to make their case against the tunnel.

In recent weeks, the University’s public affairs officials have mailed pamphlets about the tunnel to city councillors and scheduled one-on-one meetings—and even the architect who designed CGIS has personally met with councillors to sell his plan.

“We’ve been meeting with councilpeople, we’ve been talking to community groups, we’ve done mailings of materials to councilpeople and neighbors,” said Vice President for Community, Government and Public Affairs Alan J. Stone.

The personal push has even come from University President Lawrence H. Summers and, while Stone would not discuss Summers’ involvement, he did say that “discussions like these go better if they are made privately and not discussed in the press.”

Despite its lobbying efforts, the outlook remains bleak that the University will obtain digging rights, as most councillors have publicly said they oppose the tunnel.

But several councillors said yesterday they are not ready to give the tunnel an up-or-down vote yet. They say they want more time to consider the pros and cons of Harvard’s plans and possible compromises—which neighborhood activists say they will offer at tonight’s meeting.

“I’m not ready to vote for it, I know that for sure,” said councillor Henrietta Davis.

Davis said she hopes to convince the council to hire consultants to evaluate how tunnel construction would affect the neighborhood and how utilities would have to be rerouted.

And eventually, she said, the consultants’ work could be the basis for a plan that would satisfy both sides.

“It would be nice to have a neighborly settlement take place,” Davis said.

Harvard administrators have described the tunnel as a crucial piece of the project that will house the government department and a half-dozen related research centers. Last month, the city’s planning board agreed that the tunnel would make the project a better long-term development.

The board recommended that the city council grant permission for the tunnel. But in the past, the council has frequently disregarded planning board recommendations, said Cambridge political observer Robert Winters.

And he said city planning decisions are more likely to turn into political maneuvering when Harvard development projects are involved.

“Their goal is not necessarily to solve a problem,” Winters said. “It’s also their goal to reinforce their position for the next election.”

Councillors said they have received many e-mails and phone calls from both sides but they say that, on balance, more opinions have come from mid-Cambridge neighbors who oppose the tunnel.

“It’s a very ambitious project, to say the least, and would cause some major disruption to the immediate neighbors,” said Councillor Timothy J. Toomey, who raised objections to the project but said he hasn’t made up his mind to vote against the tunnel. “I always like to listen to both sides, but I’m somewhat skeptical at the moment.”

The University does not have the six votes it needs to achieve a two-thirds majority on the council in favor of granting an easement for the tunnel. But administrators say they are still hopeful that, in the end, they will win over a majority of the councillors.

“I think at the moment we do not have the votes we need, but I do feel that the case for the tunnel is strong,” said Senior Director of Community Relations Mary H. Power.

As part of an effort to make the University’s case, Dean of the Faculty Jeremy R. Knowles has been encouraging professors to attend tonight’s meeting.

“I certainly don’t want to leave the only presence, and the only audible message, to a small group of neighbors,” Knowles said yesterday.

Administrators said their attentions have been so intently focused on getting the tunnel easement approved recently that they have not developed a plan B in case the council denies tunnel-building permission.

“At this point, we do not have a contingency plan because all efforts are focused on the option we have proposed,” Associate Dean for Planning David A. Zewinski ’76 wrote in an e-mail.

The president of the Mid-Cambridge Neighborhood Association—the organization that has long opposed the government center and this winter renewed its objections to the tunnel—said his organization is willing to discuss compromises.

“We’re going to make some proposals to have something other than an up-or-down vote on the tunnel,” said association President John Pitkin.

—Staff writer Lauren R. Dorgan can be reached at dorgan@fas.harvard.edu