Newsom endorses California Proposition 42 to bar retroactive enforcement of ‘Billionaire Tax Act’

California Gov. Gavin Newsom endorsed Proposition 42 on Friday, backing a ballot measure that, if approved by voters, would prohibit retroactive taxes tied to Proposition 40, known as the Billionaire Tax Act, according to a report by The Hill.

The Billionaire Tax Act, Proposition 40, is described in the report as imposing a one-time tax on California billionaires. Proposition 42 is presented as the measure that would prevent retroactive application of the taxes associated with Proposition 40.

Newsom’s endorsement comes as California prepares for the ballot process in which voters would decide whether Proposition 42 becomes law. The report frames Proposition 42 as a step to limit how the underlying billionaire tax proposal would be applied, focusing specifically on whether tax obligations can be imposed for past periods.

Under the approach described by The Hill, Proposition 42 would act as a countermeasure to Proposition 40 by restricting retroactive taxes. The practical effect, as characterized in the report, would be to ensure that tax enforcement would not reach back in time under the billionaire-tax provisions.

The report does not detail specific enforcement mechanisms or timelines beyond the ballot decision itself, and it does not provide additional statutory language about how the retroactivity ban would be implemented. The headline question for voters, as presented, is whether Proposition 42 should be adopted to narrow the reach of Proposition 40.

The Newsom endorsement also highlights a broader state-level dispute over tax timing and due-process concerns related to retroactive taxation, with Proposition 42 positioned to limit potential retroactive financial exposure.

If Proposition 42 is approved by voters, it would become the controlling measure on whether retroactive taxes under Proposition 40 may be imposed, according to the report’s description of what the ballot proposal would do.