The California Democratic Party Sunday endorsed a ballot measure that will impose a new tax on billionaires, despite opposition from Gov. Gavin Newsom and the state's influential labor unions. 

The vote to back the measure narrowly won after two votes — one to endorse it and another to remain neutral — failed. Prior to Sunday's vote, the measure had gained few endorsements, Politico reported. 

The measure will create a one-time, 5% tax on billionaires. 

Rep. Maxine Waters rallied the party to get the endorsement passed, after a previous vote fell short of the 60% threshold needed to get the party's support. The union that spearheaded the tax measure, SEIU United Healthcare Workers West, said the endorsement shows that Democrats are lining up behind the proposal. 

Waters' support was coupled with backing from Vermont Sen. Bernie Sanders, and it seemed to energize party activists, one attendee told Politico. 

Other unions have opposed the measure, including the California Teachers Association, because, they argue, it complicates efforts to extend an income tax on the wealthy to fund schools. A measure for that tax proposal will also go to voters in November. 

Supporters, such as the SEIU-UHW, argue the billionaire tax will offset federal health care funding cuts, while opponents say it will drive wealthy residents and their tax dollars out of state.