SACRAMENTO — Champions of a project to bring BART to San Jose, connecting major East Bay, South Bay and Peninsula transit systems, celebrated a major announcement on Thursday: The extension has won a $730 million grant from the state to build it.

The final phase of the $4.8 billion extension, a 6.1-mile stretch slated to open in 2026, would bring the system through downtown San Jose and into Santa Clara with four stops along the way. With the new five-year state investment, advocates have now raised $3.65 billion of that total, a major boost for a project aimed at easing bumper-to-bumper traffic and cutting planet-warming vehicle emissions in an area notorious for its freeway traffic.

“It’s really a milestone in Silicon Valley’s transportation history,” said Sen. Jim Beall, D-San Jose, author of last year’s Senate Bill 1, which hiked gas taxes and registration fees to raise $5 billion annually for transportation improvements.

The news does come with a significant caveat: Revenue from Senate Bill 1, the largest source of the grant money awarded Thursday, will dry up if opponents succeed in repealing the tax and fee increases through a ballot initiative in November.

“I have an answer for you: Poof! It all goes away,” Beall said when asked about the implications of a successful repeal campaign.

Also on Thursday, the BART board of directors officially approved a tunneling method for the project, putting to rest a dispute with Santa Clara Valley Transportation Authority. The plan is to dig one tunnel instead of two. The approach, expected to be quicker and less disruptive, has yet to be used for a passenger rail project in the United States.

The BART extension still needs to secure federal funding, a request it plans to make to the U.S. Department of Transportation later this year. Its supporters also hope to raise $375 million from a Bay Area-wide June ballot measure, known as Regional Measure 3, that would gradually raise most Bay Area bridge tolls by $3.

Heading the bridge-toll campaign is the Silicon Valley Leadership Group, a business trade association that has worked for decades to piece together the necessary funding for the BART extension, pushing through three local ballot measures since 2000.

“None of this was for the faint of heart,” the association’s CEO, Carl Guardino, said in an interview Thursday.

The extension was one of 47 projects across the state vying for a $2.6 billion pot of grant money from the California State Transportation Agency to expand light rail, zero-emission buses and other public transit systems. The state also awarded funding to add zero-emission AC Transit buses, more rail cars for BART and SF Muni, and a SamTrans express bus route along the clogged Highway 101.

The BART extension project was awarded every penny it requested from the state’s Transit and Intercity Rail Capital Program, a pot of money made up of new vehicle registration fees and proceeds from California’s cap-and-trade program. It was one of seven projects that secured multi-year agreements, bringing the total funding announced Thursday to $4.3 billion.

“Folks, it’s real now,” Sen. Bob Wieckowski, D-Fremont, said Thursday. “This little tranche of money makes BART to Santa Clara, to downtown San Jose, real.”

But the specter of the gas-tax repeal cast a shadow over an otherwise celebratory moment for transit advocates. As the news was announced, a repeal campaign led by San Diego talk-radio host Carl DeMaio was preparing to deliver more than 900,000 signatures to election officials for review — well over twice the number needed to qualify for the November ballot.

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VTA, BART agree on one tunnel option for San Jose extension At Thursday’s news conference, Beall suggested the repeal initiative was a get-out-the-vote strategy for the Republican Party ahead of the midterm elections.

“I think it would be ridiculous if the voters took heed of these short-sighted people that have for political purposes proposed this initiative,” the senator said. “The initiative’s intention, in a deep-down way, is more about saving their skins in the election than it is stopping these projects.”

Reached by phone, DeMaio said it was “disrespectful” to characterize the drive as a partisan scheme. He argued that the campaign he started has won support of people from all political backgrounds who object to the 12-cent gas tax increase and higher vehicle registration fees that have taken effect in recent months.

“Californians don’t want to pay the higher tax for their car or their gas,” DeMaio said. “It’s as simple as that.”