It's a weird effect of the two-party system that neither party ever gets a chance to actually implement their ideas and programs fully without being hamstrung in some way by the other party, so that it's actually impossible to see what the country would look like if either Republicans or Democrats got full run of the board.

So like, you get the ACA, which was not nearly as strong or sweeping or as beneficial as it could have been if not for having to compromise with Republicans, and sure enough the Republicans then point to it and say "the ACA isn't working!" Ditto gun control, environmental protections, etc etc.

The last couple of years seem to be the closest thing in recent memory to seeing what it's like when one party gets everything it wants, and...what did you get? Multiple government shutdowns, a crashing failure to repeal the ACA (which they've been promising to do since it passed), and one of the least legislatively productive Congresses in decades.