If the United States of America had to change from having a federal government into either a
*Unitary government Similar but not identical to the UK/Poland/Israel/France/Japan/Norway/Sweden/Finland/NewZealand/Chile/Spain/Portugal/Italy and the 150+ other unitary governments in the world
The national/unitary/central government can grant autonomy to subdivisions, but also can take them away and take direct control, change internal boundaries and subdivisions however they want, and can also define and change their relationship to those subdivisions/states however it wants
States only have the authority and power that the national/unitary government gives them, national law supersedes all state/local laws, no more states' rights
Wikipedia:
*also, the Senate is
probably eliminated in this scenario
or
*Confederate governmentSimilar but not identical to the pre-Constitution Confederation, or the Confederate States of America, or the European Union, or the United Nations.
The national/confederal government can be responsible for some national defense, a common currency, freedom of movement and trade within, maintain borders with non-members, conduct diplomacy... and not much more
[no strong chief executive, very little taxing power, no national court system since the Congress/Senate would be the ultimate decider in the little legal area they have, and only allowed to go into debt in times of war]
Much more consensus-driven or possibly even unanimity needed, or no decisions and the possibility of a member to peacefully leave
At the national/confederal level: no Social Security, Medicare, Medicaid, EPA, Food Stamps, economic stimulus, mandatory desegregation or religious disestablishment or even the bill of rights, neither pro-life/pro-choice or pro/anti-SSM at this level... etcetera. But states are free to choose however they wish, with each state more like its own country.
States can certainly try those things and even more "progressive" projects
(but it might be hard for the smaller states who want grand social projects, like single-payer healthcare, to actually afford it)
Wikipedia:
*also, the House of Representatives is
probably eliminated in this scenario (and it's probably up to each state in how its Senator is chosen, whether elected or chosen by state legislature or appointed by governor)
...which would you prefer for the United States in the very-near-future, if these two are the only choices?