month on a new $185 million main library in downtown San Diego despite concerns that the project could leave taxpayers on the hook should private donors fail to raise enough money to pay for it.

The City Council voted 6-2 Monday to move forward with library construction under the promise that a fundraising campaign will be able to collect the additional $32.5 million needed to finish the job. If donors don’t emerge, the city would have to either use taxpayer money to fill the gap or leave the library’s interior unfinished.

The financial risk to taxpayers was the chief concern among opponents, including former City Attorney Michael Aguirre, who questioned the wisdom of committing to a brand-new library when the city has been forced to make significant cuts to public safety, parks, beach maintenance and existing libraries. But it wasn’t enough to dissuade a council majority from proceeding.

Library supporters, including former Qualcomm founder Irwin Jacobs, who has pledged $20 million to the project, promised the money would be raised to complete the project. While the city is taking a $32.5 million risk, they said the city would lose $80 million in outside money committed to the project if it didn’t approve the plan Monday.

Former professional basketball player Bill Walton, a San Diego native whose mother had a 21-year career as a city librarian, said there is no greater mission and no grander goal than building a new library.

“The argument against this library is that it’s too expense and that we can’t afford it,” Walton said. “In reality, we can’t afford not to do this. We can either start building this library today or start building prisons tomorrow.”

Aguirre, who lost a re-election bid in 2008, said the city doesn’t have enough money to keep its current libraries open and questioned whether it was legal for the city to start construction on a project that wasn’t fully funded. He also criticized Jacobs for his donation, a comment that drew boos from the crowd.

“You’re hurting our city because you are helping to push this city into doing something that is financially irresponsible,” Aguirre said.

Aguirre said the city was heading down the path to another financial problem as it did with the Chargers ticket guarantee. The city’s past deal with the Chargers called for taxpayers to ensure the sale of 60,000 seats for the team’s home games at Qualcomm Stadium. The deal cost the city $36.4 million between 1995 and 2003.

The new library would be built at the corner of Park Boulevard and J Street in the East Village. It would be a nine-story domed centerpiece and twice the size of the existing 1954 library on E Street. Two floors would be used for a charter school serving about 400 students.

Funding for the library would come from several sources, including $80 million in city redevelopment funds, $20 million from the San Diego Unified School District and a $20 million state grant.

The rest would be covered by private donors, who have already pledged nearly $41 million. That includes $10 million to pay for the library’s operating costs for the first five years.

The city has enough money for the first phase of the library but needs an additional $32.5 million by January 2012 to start the second and final phase. The library is projected to open in July 2013.

Council members Carl DeMaio and Sherri Lightner voted against the project, saying the city couldn’t afford to build a central library at a time when the 35 branch libraries have seen their hours reduced significantly because of budget cuts.

“I’m afraid this project contains too many risks to our city’s general fund, too many shoes that may drop,” DeMaio said. “We cannot afford to build this project at this time. We have to get our city’s financial problems fixed first. We have to make sure that when we invest in our library system that all branches benefit equally.”

The project needed six votes for approval and Frye, who had opposed the library in the past, provided the swing vote to win passage. She didn’t explain her vote during the hearing.