Does the constitution guarantee that the government will function as a republic?
The constitution requires it in Article 4, section 4: “The United States shall guarantee to every State in this Union a Republican Form of Government.” The constitution can’t guarantee it, though. It’s just a document, a record of an agreement reached two hundred years ago. It’s up to the voters to insist that their elected officials continue to abide by that agreement. It’s has checks and balances in the structure of the government, but if everyone is willing to look the other way, it is fundamentally nothing more than a piece of paper.