Newsom doesn't have the balls. This is all just posturing.
There is no mechanism for California to simply "cut off" funds from California citizens to the federal government. The IRS collects tax money directly. How exactly are you going to "cut off" these flows?
Really, the only way to do it would be:
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Newsom announces that Californians don't need to pay federal income taxes, and that the state of California will directly protect its citizens from the federal government.
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Encourage and teach California citizens how to set their federal tax withholding to zero and encourage California businesses to do the same.
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Order California troops and law enforcement to throw all federal law enforcement out of the state at gunpoint.
That's what this would actually take. The feds have way more guns than any citizen can resist. The only actual way for California to stop the feds from taxing their citizen would be to directly protect the citizens of California from tax prosecution by the IRS. And that would require physically expelling federal law enforcement agents from the state. And that would likely require shooting a fair number of them.
There's no way to do this without starting a civil war. And something tells me a moderate like Newsom can't even process the idea of having the state actually secede from the union. Is a Republican-lite like Newsom going to order California law enforcement to kill IRS and ICE agents? In what universe do you see that happening?
This is all just rhetorical masturbation.