There seems to be some confusion that non-essential means not necessary. .
After the shutdown, as is always the case, agencies will choose to try to be more efficient as companies do after a strike. There will be legal efforts to reduce the numbers ordered by agency heads (after Trump demands that to be the case).
Trump and his folks will point out that there should be no great outcry in eliminating non-essential employees.
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Of course, there will be illegal firings (the courts will adjudicate these), but all that is simply diversion as with the DOGE firings (which amounted to relatively few actual firings.
Clinton/Gore reduced force by 370K or more. Trump wants to do more and he is only about half-way to Clinton numbers. Trump wants to disable and cut whole departments, as Reagan's folks campaigned on so long ago.
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Trump's folks are all about diversion from the very obvious goal of doing all they can to greatly reduce the role of the federal government and to greatly reduce non-defense discretionary spending.
Defense spending is another matter. There, Trump wants to change the focus from 20th century antiquated Department of Defense toward a more modern Defense (the iron shield against incoming missiles), a modernization of the bureaucracy (using AI and modern computers), and drones instead of new fighter jets. This could cost more or less. Obviously, the supply chain and costs within the Defense bureaucracy needs to be scrutinized to say many billions, but that is an issue for another day.
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A CAUTION
I think that Democrats make a great mistake if they campaign against the idea of reducing the role of the federal government and against a reduced number of federal employees.