You can be quite confident that if a Republican president had enacted the same policy, the same R justices would have upheld it as entirely within executive authority.
And that's the mess of this court in a nutshell.
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Remember when TFG (aka the Orange Stain) diverted federal money appropriated by Congress for other purposes for his "wall," under a ludicrously strained reading of federal law?
The court upheld that. They wouldn't have if a D had done the same thing. Just as the court would have upheld an identical debt forgiveness from TFG if he'd done it (unlikely though that is - just imagine he decided to try to buy youth votes).