A union that represents BART employees is waging an unlikely campaign. Its goal: to defeat a Contra Costa County ballot measure that would fund BART and other transportation projects.

Measure J aims to raise $103 million annually over 35 years by increasing sales taxes by a half cent throughout the county, spending the money on new eBART cars for the line from Pittsburg to Antioch, ferry service, buses, infrastructure to “harden” BART stations against fare evaders, traffic-light synching along major roads, new bicycle routes and, possibly, an extension of the eBART line to Brentwood.

Transportation officials, politicians and environmental groups that back the measure are scrambling to rally support from two-thirds of voters Tuesday, a challenge made more formidable by sudden opposition from Service Employees International Union Local 1021, the union that represents mechanics and clerical workers at BART.

The union said it would support the tax only if BART agreed to renegotiate its contract and fire top managers of the eBART line — demands that BART did not accept, according to a correspondence obtained by The Chronicle.

Last month, the SEIU spent $46,546 to mail flyers throughout the county, each showing pictures of giant, noxious smoke clouds.

“The train is coming,” the flyers said, “and it is spewing 22,000 pounds of pollution everyday.”

Union President John Arantes said his members have environmental objections to eBART, which runs on biodiesel fuel while the rest of the system is powered by electricity.

“We think this is discrimination, because a lot of people in Antioch are low income,” Arantes said. “Antioch (residents) are paying taxes, but they’re getting a substandard system.”

Yet he acknowledged that SEIU has another reason to oppose eBART. Workers on that segment of the rail system have a separate labor contract from their colleagues in the core BART system. It’s been a bitter point of contention since before BART opened the Antioch line in 2018.

“They have a second-tier contract, which means they don’t have the same rights,” Arantes said. “We’ve accused the (BART) district (management) of safety issues and discriminating against minorities. We’ve been continuously fighting over a lot of issues over there.”

That fighting came to a head with Measure J, which has driven a wedge between the union and BART officials, who broadly advocate for the measure. Deputy General Manager Michael Jones told The Chronicle he’s flummoxed by the union’s opposition.

“I have no idea why,” Jones said, pointing to all the benefits the sales tax would provide, including expansion of the fleet that would produce more union jobs.

But in a memo forwarded to The Chronicle, Jones accepted four of the union’s demands: to hold eBART hearings during day-shift hours, issue a statement against workplace bias, stop using GPS tracking locators to discipline employees and release a statement highlighting BART’s commitment to prevent safety violations.

Jones and Tamar Allen, BART’s assistant general manager of operations, refused the last two demands, to fire management officials at eBART, and to place eBART employees on the same labor contract as other BART employees.

They presented the deal to Arantes on Feb. 4. Arantes responded with an angry email the following day.

“Since the district is not serious about resolving the ongoing issues at eBART, SEIU is pulling all support of Measure J, and we will be raising objections to any future funding initiatives that contribute to the expansion of eBART diesel operations, and the procurement of additional diesel BART trains in Contra Costa County,” he wrote.

Days later, the first union flyers showed up in people’s mailboxes.

Environmentalists lobbying for Measure J say its improvements to public transportation would far outweigh the harmful effects of biodiesel emissions.

“The money for eBART is a very small portion — 0.8 % — of the funds, compared to the amount for buses, bicycle and pedestrian infrastructure, improving bottlenecks on the freeway and making BART cleaner and safer,” said Hayley Currier, policy advocacy manager for the nonprofit group TransForm. She pointed to two key goals of Measure J: to cut back 15% of car vehicle miles by 2055 and reduce greenhouse gas emissions by 80% over the same time period.

Opponents of Measure J, including taxpayer advocates and government watchdogs, argue that it would devote millions of tax dollars to mass transit, but only marginally discourage solo driving.

“It’s a fantasy that this is going to relieve congestion,” said David Schonbrunn, a member of the campaign to reject Measure J.

Some officials at BART see SEIU’s campaign as a form of political posturing.

BART Board Director Debora Allen, whose district encompasses central Contra Costa County, accused the union of using its opposition to Measure J as a cudgel to reopen negotiations for the eBART labor contract, which expires in 2022. Union leaders offered to support Measure J if BART agreed to renegotiate the contract, she said.

“I think it’s outrageous that the SEIU Local 1021 opposed Measure J after BART management refused to reopen existing labor contracts in exchange for their support,” Allen said in an emailed statement. She praised the agency for standing up “to this ridiculous move by SEIU to coerce contract changes in exchange for its support of a ballot measure that ultimately funds the paychecks of SEIU’s own members.”

State Sen. Steve Glazer, D-Orinda, also criticized the SEIU for “militant and self-destructive actions against a system they profess to support.” Glazer is a reliable critic of BART and its labor unions.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan