BART Board President Melissa Hernandez and state Sen. Aisha Wahab are facing off in the race, with public safety emerging as a central issue and millions of dollars flooding into the contest from both campaigns and outside groups.
The race comes after Wahab won the June primary by 26 percentage points, but a surge of spending has intensified the fight in its final stretch.
Federal campaign finance filings show Hernandez raised about $1.53 million and spent roughly $1.35 million between Jan. 1 and July 29, according to Federal Election Commission records.
During the same period, Wahab raised about $631,000 and spent about $484,000.
Outside groups have spent even more. Super PAC spending has topped $5 million on ads supporting Hernandez and opposing Wahab. Independent expenditure filings show nearly $2.9 million in year-to-date spending from known AIPAC-affiliated sources backing Hernandez or opposing Wahab.
MORE: Alameda County voters to decide interim, full-term replacement for ex-Rep. Swalwell
Political consultant Jim Ross said the spending reflects a broader national strategy.
"They want to send a message to other candidates, other people running for office, that we will spend an enormous amount of money against you if you oppose Israel and the ongoing military support for Israel," Ross said.
Wahab has sharply criticized Hernandez's campaign and the advertising campaign against her.
"Our district is not a district that can be bought, and she will eventually figure that out," Wahab told ABC7.
"Her strategy has been to just mudslinging to bring me down on false or inaccurate misleading statements and commercials and ads."
Wahab has also defended her record on public safety, pointing to legislation she has supported in Sacramento.
"I am a coauthor of SB 14 that made human trafficking a felony. I have voted to ensure that solicitation of minors is a felony," Wahab said. "My father was murdered. I know what it means to prioritize victims, because that crime is not just to that one individual, but literally it has lasting effects on their family."
MORE: Rep. Eric Swalwell resigns from Congress after sexual assault allegations
Hernandez, meanwhile, has leaned into a law-and-order message and highlighted new law enforcement endorsements during an event in Pleasanton on Monday.
"I feel like I'm tougher on crime," Hernandez told ABC7. "We want to make sure that bad actors are held accountable."
She has also sought to portray Wahab as part of the political establishment.
"My opponent has spent three years in Sacramento. So let me ask you a straightforward question. Has Sacramento been working for you?" Hernandez said.
"Sending more Sacramento politics to Washington, D.C. isn't change."
Regardless of the outcome of the Aug. 18 special election, Hernandez and Wahab will face each other again in November in the race for the district's full congressional term.