SAN FRANSISCO, Nov 9 (Reuters) - A billionaire tax that US Senator Bernie Sanders ​and other progressives championed as an antidote to economic inequality and a potential model for the ‌rest of the country was defeated in California, US media projected on Monday.

Proponents had estimated Prop 40 would raise $100 billion in revenue, most of which would have been earmarked for healthcare programs to offset federal funding cuts designed to pay for tax breaks enacted ​last year under President Donald Trump's "One Big Beautiful Bill." Those tax breaks, largely benefiting wealthy individuals and corporate ​interests, passed with Republican-only support in the US Senate.

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The failure of the billionaire tax marks ⁠a blow to the left wing of the Democratic Party, even as Democrats won decisive majorities in both ​houses of Congress in a sweeping midterm election rebuke of Trump and his policies.

Supporters of Prop 40, which targeted stocks, ​bonds and business assets rather than earned income, reasoned it stood a healthy chance of enactment in a state where voter rolls favor Democrats over Republicans by a nearly two-to-one margin.

But California Governor Gavin Newsom, a likely candidate for the Democratic Party presidential nomination ​in 2028, came out against the measure, arguing that Prop 40 would backfire in the long run by creating ​an unwelcome climate for upwardly mobile high-tech entrepreneurs.

His opposition and division among party leaders led initial support for the measure to wane ‌ahead ⁠of the vote.

Opponents argued the tax would prompt the ultrawealthy to leave the state, taking their income-tax payments and job-producing enterprises with them, and discourage entrepreneurs from building their startups in California in the first place.

Labor unions were also divided. Billionaires chipped in more than a quarter of a billion dollars that helped fund ads that framed the measure ​as a gateway to more ​aggressive taxation for all ⁠Californians and as a threat to their retirement accounts.

Those arguments may have resonated at a time when the pinch of higher gas prices and affordability concerns have put a ​broad swathe of the country in a sour economic mood.

Google co-founder Sergey Brin and at ​least 11 other ⁠billionaires contributed a collective $269 million to a political action committee called Building a Better California, which in turn poured more than $300 million into campaigns opposing Prop 40.

Incoming Senate Finance Committee Chairman Ron Wyden has signaled, however, that taxing the ultrawealthy remains a ⁠central goal ​for the Democratic Party as a whole.

“The committee will redouble its ​efforts to ensure billionaires pay their fair share and hold tax cheats accountable,” he said in a November 4 statement outlining the panel's priorities.

Reporting ​by Ann Saphir and Peter Henderson; Additional reporting by Jasper Ward; Editing by Edward Tobin, Caitlin Webber and Edwina Gibbs