Super Pacs tied to the American Israel Public Affairs Committee (Aipac) have spent millions in a California congressional race to keep a progressive Democrat from winning Eric Swalwell’s former seat.

Aisha Wahab, a state senator who has been endorsed by the California Democratic party, is widely considered the frontrunner in the special election for the district covering a swathe of the East Bay.

Swalwell had been in office for more than a decade when he resigned earlier this year following multiple allegations of sexual assault.

Wahab outpaced her closest competitor, Melissa Hernandez, a former Dublin mayor, in a June primary as well as a special election to complete the remainder of Swalwell’s term, by more than 20 points. They are set to face off against each other in a runoff election later this month, and then again in November.

Super Pacs are pouring money into the race, dropping more than $5m in recent weeks and supporting pro-Hernandez ads and Wahab attack ads, Politico reported on Thursday. The spending includes $1.2m from United Democracy Project, which is backed by Aipac, and $2.1m from Bold America Pac, which has received significant financial backing from the United Democracy Project, according to the outlet.

Track Aipac, which tracks the group’s funding of US politicians, reported that the lobby has spent more than $4.4m to support Hernandez in the race.

Aipac, which is one of the largest Super Pacs and the most powerful pro-Israel lobby organization in the US, has significantly expanded its political spending in recent years. The New York Times reported earlier this week that Aipac has doubled its spending, largely in Democratic primaries, to $67.2m.

The organization spent millions to support moderate Haley Stevens in Michigan’s Democratic senate primary. The progressive Abdul El-Sayed ultimately won the primary race in what has been described as a defining contest for the party.

Wahab recently spoke about the negative advertisements against her, telling voters to come to her with questions about her record. She criticized Hernandez for allegedly dodging debates while “relying on dark money” to support her campaign.

“We’re not going to let this race be bought by outside interests,” Wahab said in a video posted on social media.

The Guardian has contacted Aipac, as well as the campaigns of Wahab and Hernandez for comment.