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Too complex an issue to answer properly in a short way.

That said, the short answer is that Marbury v. Madison vested ultimate power to resolve constitutional issues in the Supreme Court. There’s no way for the President and/or Congress to override a Supreme Court decision. But, not all issues get to the Supreme Court.

So, as a day to day practical matter, Democrats controlling the Executive and Legislative branches would have significant control/power.... but the Supreme Court would have the ultimate checkmate on issues that get all the way up to the SC.