Blue States should write legislation to withhold federal taxes

[adnoid]

5. Exactly this

What happens when the DOJ charges either those employers or the individual taxpayers with tax evasion? Do constituents of these states want their Social Security accounts no longer being funded ?

This is exactly the point. The employers and individuals owe these funds to the US Treasury. The State would have to confiscate them and keep them - would they intercept the bank transfers or something similar? I don't see how this would relieve the payers of the responsibility to the Treasury.

When your employer withholds income and SS amounts from your paycheck, it's not the employer's money - it's your money, the employer is legally required to send it to the Treasury on your behalf. They don't send it, you don't get credit for it, and it's still owed.

Would the State tell employers, for instance, that the State would indemnify them from collection efforts, penalties, interest, etc? That would get very expensive for the State.