1. Cannot approve any laws:
He needs Congress. With Democrats at 90, all his legislative proposals will be blocked.
2. Cannot secure funding:
Congress controls the budget. No financing for his MAGA projects, the Wall, etc.
3. Faces the risk of impeachment: Democrats can launch investigations and even proceed with impeachment.
4. Cannot appoint judges/officials freely: The Senate will block his choices.
5. Cannot conclude major foreign deals:
Trade agreements, war funding require Congress's approval.
In short: He will remain in the White House, sign executive orders, but Congress and Courts can overturn them. President in name, powerless in reality.