The Golden State Warriors’ plans for an 18,000-seat arena in San Francisco’s Mission Bay are suddenly running into big-time political problems.

An anonymous group of what organizers describe as big-bucks donors to UCSF hired an imposing cast of consultants — including former UCSF Senior Vice Chancellor Bruce Spaulding and, for a time, Chronicle columnist and former Mayor Willie Brown — to block the plan for the arena and adjacent twin office towers in Mission Bay near the waterfront.

Also on board, and working without pay: Jack Davis, once the biggest political consultant in town and still a force to be reckoned with in semi-retirement.

“This arena is going to essentially ruin decades of good work and planning in Mission Bay and make it impossible for people to access the hospital there,” said public relations pro Sam Singer, who has also been hired by the antiarena forces.

The emergence of the opposition group comes just a month before the final environmental impact report for the Warriors’ arena is due to be released — raising suggestions that the effort is largely aimed at trying to force the team to scale back its ambitious plan for developing 12 acres next to UCSF.

Along with the arena, the project would include two 160-foot-tall office buildings and a central plaza larger than Union Square.

One big issue, said Singer: The Warriors plan to build just 950 parking spaces for the project, 650 of which would serve the two office towers.

Singer called the parking figures “absolutely ludicrous.”

Spaulding, who spent years developing the plans for the Mission Bay medical campus, says the Warriors have shown “no serious willingness” to negotiate the terms of the project — including plans to host as many as 200 events a year at the arena.

“The Warriors are articulating that they need the whole shebang to make the economics work,” Spaulding said — leaving his group in “the unfortunate position” of having to take it or leave it.

“I think there really is a preference for a different location,” Spaulding said.

Anonymous nonprofit

The opponents have formed Mission Bay Alliance, a nonprofit whose contributors are allowed to remain anonymous.

Singer wouldn’t say who was financing the effort, but said alliance board members include Chiron founder William Rutter, Dr. Samuel Barondes, former chairman of the Langley Porter Institute, and retired UC Hastings law Professor Richard Snyder.

Although Spaulding stopped short of saying the group was prepared to take its case to the voters — a la the “No Wall on the Waterfront” effort that knocked off the 8 Washington condo development — Singer said that was “absolutely” a possibility.

The first step, however, may be putting up a legal challenge to the environmental impact report — a move that could delay the Warriors’ plans to break ground next year.

Brown at first was on board with opponents, but decided Tuesday to back out because his involvement could pose a conflict of interest with his status as a weekly Chronicle columnist.

Before dropping out, he said his intention was not to kill the arena. “We want to make it more amenable to the neighborhood,” he said.

Warriors spokesman P.J. Johnston, who used to be press secretary for Brown, questioned the validity of the opposition group.

“It’s hard to know who or what this shadowy new organization may be, but they don’t appear to represent UCSF, and certainly not the community,” Johnston said. “The San Francisco public overwhelmingly supports the Warriors’ move to Mission Bay.”

Mayor Ed Lee’s press secretary, Christine Falvey, said the Warriors’ arena “has broad support across the city and in the Mission Bay neighborhood, including from UCSF.”

Falvey added that “the city is working with the hospital directly and has already addressed dozens of issues to make sure we can have a world-class hospital and a world-class arena. ... We are moving forward and making a lot of progress.”

Any serious public opposition to the Warriors’ jump across the bay had seemed to fade away last year after team owners Joe Lacob and Peter Guber dropped plans to build on the waterfront at Piers 30-32 and focused instead on Salesforce-owned property in Mission Bay.

But behind the scenes, UCSF brass and benefactors have remained divided over the deal — and, at the very least, skeptical.

In a confidential Feb. 12 letter to Lee and the Warriors front office, UCSF Chancellor Sam Hawgood called on the team to remedy “what we identify as the emerging critical conditions” with the arena project, mainly parking and traffic.

Ambulance concerns

Hawgood’s worries included that the development could snag ambulances in gridlock, especially when the Giants are at home at the same time an event is being held at the arena.

Hawgood hinted that unless “certain reasonable conditions” were met, UCSF might use its leverage to block the deal — namely “an enforceable and binding view easement” over a portion of the arena site protecting the hospital’s views.

Lee wrote back that “monitoring traffic conditions, determining priority access routes and constructing new ... parking structures on port property are issues for the city to resolve.”

And in a hint that things could get ugly for the hospital, he threatened to go after UCSF to pay its fair share of “transportation and utilities network” costs.

“We continue to have concerns,” Lee wrote, about the medical complex’s exemption under state law from local property taxes, parking taxes or transportation impact fees.

Since then, however, we’re told there have been negotiations between the city and UCSF that have brought the two sides closer together.

“We have been pleased with the city’s response, and how much they have been doing to try to mitigate traffic and parking congestion,” UCSF Vice Chancellor Barbara French told us Tuesday. “We continue to look forward to working with them and the Warriors to make the deal work.”

But French added, “We have to stand up and support public safety and patient care.”

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or e-mail matierandross@sfchronicle.com. Twitter: @matierandross