The well-coiffed Democrat -- seen as a potential 2028 presidential candidate -- slammed Trump for leaving the Paris climate accord and for "doubling down on stupid" through his pro-oil stance.
Newsom said a Democratic administration would rejoin the Paris Agreement "without hesitation."
"It's a moral commitment, it's an economic imperative, it's both -- and it's an abomination that he has twice, not once, pulled away from the accords," Newsom said in response to a question by AFP in Belem, the Brazilian Amazon city hosting the climate summit known as COP30.
Newsom appeared alongside Helder Barbalho, the governor of Para state, of which Belem is the capital, at an event on the city's docks.
Between bites of cupuacu fruit and sips of acai juice, he touted California's green credentials, noting that the state now gets two-thirds of its electricity from renewable sources.
It was the first stop for the leader of the world's fourth largest economy, followed by a signing ceremony with the German state of Baden-Wurttemberg, and a press conference with the Brazilian's summit leadership among events to follow.
Trump withdrew the United States from the Paris climate accord -- for the second time, after doing so in his first term, too -- upon returning to office in January and has sneered at the idea of human-caused planetary warming, calling it a "con job".
But Champa Patel, executive director for governments and policy at Climate Group, which runs the Under2 Coalition of global states and regions, said US states could still pursue the climate blueprints left by former president Joe Biden's administration.
"The states have that roadmap, they can still follow it and keep to the spirit of Paris," Patel told AFP, adding it was important to signal solidarity.
"Even if national governments backslide, or if they undermine their own commitments, subnational governments, cities, states, regions, are really at the vanguard of implementation."
Still, their influence has limits: subnational regions have no seat at the negotiating table at COP30, which opened Monday with urgent calls to stay the course on climate action.
"The absence of the United States of America does not compromise the COP," Portuguese Prime Minister Luis Montenegro told reporters last week, adding Portugal would like for "the United States of America to return to these objectives."
- State power -
"The president can't throw a switch and turn everything off -- that's not how our system works," added Nate Hultman, a former official in the Barack Obama and Biden administrations who now works as a researcher at the University of Maryland's Center for Global Sustainability.
A recent analysis by the group found that if leading states and cities enhance action -- and if a climate-friendly president is elected in 2028 -- US emissions could fall by well over 50 percent by 2035, approaching the 61-66 percent targeted by Biden's government.
Much of that stems from states' authority over energy and building policy, and cities' control of waste management, methane reduction, public transport, and more, Hultman told AFP.
The market-driven green shift remains a strong factor including in US states with climate-hostile leadership, including Texas, the country's renewable energy generation leader last year.
New Mexico's Michelle Lujan Grisham, another Democratic governor, is also at COP30. She governs a major fossil fuel-producing state but has pushed to expand renewables and curb methane emissions from the oil and gas sector.
Reflecting America's divided politics on climate, pro-oil and gas group Power the Future slammed the New Mexico leader for "packing her bags for another international climate junket."
Still, questions linger over the limits of state-level action.
Trump's Republicans recently passed a law bringing an early end to clean energy tax credits enacted under Biden. This is seen as a potentially crippling blow to the renewable sector.
His administration also torpedoed international efforts to impose a carbon tax on shipping, vowing reprisals against countries that backed the plan.
Newsom urged nations to hold firm, saying it was vital to remember "Trump is temporary" and that "you stand up to a bully."
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