Note that it would have to be a State Guard that is part of state law enforcement, not a National Guard that is part of the US military. But yes, it is is possible, though it's a first step to breaking up the United States, which Russia would be overjoyed by.
Yup, the issue is that the National Guard can be federalized so would have to be outside of the US military. Pretty much the only way that I can see this working is by structurally creating it as a state agency separate from the military under the guise of a state law enforcement agency.
Not that I'm a fan of the notion of creating state paramilitaries but here we are.
Theoretically, a governor could activate the national guard first to support the constitution and state residents. They could argue that they were in use and were not available for federal duty. The feds would have to file suit to argue that they needed them for action AGAINST state residents.
I mean these are all hypothetical scenarios but i think this is a case of avoiding the legal challenges. Think in terms of chain of command, etc.