In a dramatic reversal of its stance just six months ago, Silicon Valley’s largest water district has scheduled a vote Wednesday on a plan to commit up to $650 million to Gov. Jerry Brown’s controversial proposal to build two massive tunnels under the Sacramento-San Joaquin River Delta.

The shift comes amid questions over whether the Santa Clara Valley Water District struck a deal with the Brown administration to support the tunnels plan in exchange for the state funding a new dam that the San Jose-based agency wants to build near Pacheco Pass.

Brown’s tunnels project — one of his two legacy projects along with high-speed rail — aims to make it easier to move water south.

In February, the staff of the California Water Commission, whose nine members are appointed by the governor, recommended against funding the Pacheco dam project. Last week, however, after officials from the water district held discussions with the Brown administration on the tunnels, the water commission’s staff recommended the full $485 million in funding for the Pacheco project that the water district had been seeking.

Dick Santos, chairman of the Santa Clara Valley Water District, said Friday that he isn’t sure whether he will support the district funding the tunnels.

“I haven’t changed my mind, but I’m willing to discuss the principles if people aren’t on the hook for increased costs,” he said. “I’ve got to listen. Do the benefits outweigh the costs?”

Santos said he hasn’t heard whether the agency’s staff discussed the Pacheco Reservoir funding as part of its talks on the tunnels with the Brown administration officials, who have lobbied the district heavily, as has the governor, to help fund the tunnels.

“That’s easy to assume,” Santos said. “But I haven’t heard that there was that kind of a threat or offer to us.”

Critics of the tunnels project, however, called the timing suspicious.

“It is interesting to note that Santa Clara’s renewed interest is at the same time that the governor’s water commission has just determined that Santa Clara is eligible to receive $485 million to build a new dam,” said Jonas Minton, senior water policy adviser at the Planning and Conservation League, a Sacramento environmental group. “Because all of these discussions have been behind closed doors, there’s no way to confirm the suspicion that this was a back-room deal.”

Minton, a former deputy director of the state Department of Water Resources, noted that if the Santa Clara Valley Water District agrees to help pay for the tunnels, homeowners in Santa Clara County could be hit with higher property taxes without a vote if the project has cost overruns, similar to the high speed rail and the Bay Bridge rebuilding projects. That’s because under a legal loophole, the tunnels project is part of the State Water Project, which voters endorsed in 1960, and which pre-dates Proposition 13, the 1978 measure that requires a two-thirds vote for most property tax increases.

“Governor Brown, who is in the middle of all of this, is trying to rush through a half-baked project before his term ends in eight months,” Minton said. “Next week’s decision by the Santa Clara Valley Water District Board would obligate ratepayers and property taxpayers for decades to come.”

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In a staff report, Garth Hall, deputy director of the water district, said the project would raise the water rates of customers in the urban areas of northern Santa Clara County by up to $10.26 a month by 2033, when the tunnels would begin operations, and $4.47 a month in the southern part of the county.

Two years ago, a majority of the district’s board said they wanted to put the tunnels question before Santa Clara County voters for a vote. No vote is scheduled, however.

The project, which Brown’s administration has called WaterFix, would build two tunnels, each 35 miles long and 40 feet high, at a cost estimated at $17 billion, to be paid by water agencies that participate. The idea is that they would take water from the Sacramento River, 17 miles south of Sacramento, and move it to the huge pumps at Tracy that are part of the State Water Project, reducing reliance on those pumps.

At certain times of the year, when salmon, Delta smelt and other endangered fish swim near those pumps, courts have ruled that the pumps be turned down, or off temporarily, so they don’t kill the fish.

But critics say that the tunnels are a boondoggle that would allow large agribusiness interests in the San Joaquin Valley, as well as urban users in Los Angeles, to take more water out of the Delta, a fragile system of islands and sloughs. They maintain that would not only be a water grab from Northern California but would potentially degrade the Delta and San Francisco Bay, its wildlife and water quality.

The project had been struggling for cash after Westlands Water District, a large farm water agency in Fresno, backed out of contributing $3 billion, citing high costs and debt risk.

But earlier this month, the Metropolitan Water District of Southern California, which serves 20 million people in Los Angeles and other counties, increased its commitment to $11 billion, giving it new momentum.

In his staff report, Hall said, that change makes the project more viable and worth another look by Santa Clara Valley Water District. The meeting is scheduled for 5 p.m. Wednesday at the district’s headquarters at 5750 Almaden Expressway in San Jose.

Hall is also overseeing the district’s Pacheco Reservoir project. On Friday, he referred questions to the agency’s public affairs office.

The district’s communications director, Linh Hoang, did not make Hall or district CEO Norma Camacho available for questions. Instead, she sent out an emailed statement from a board member, Tony Estremera.

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“There have been some significant developments recently on the California WaterFix project, so we are looking forward to staff providing us with an update as well as with information on the agreements and different approaches to participation,” Estremera said in the statement.

“Our board will determine how the agreements and participation tie into our guiding principles, and what this means for our valley, as the full project is before us for consideration. We are still aiming to achieve the best outcome for Santa Clara County, and that includes investing in infrastructure to ensure our water supply for the future, while also doing our part to protect the Delta environment.”