It's more of a polyarchy. The U.S. system is unique (originally but unremarkable today) in two respects: written constitution, and not a monarchy. Ambition must

It's more of a polyarchy. The U.S. system is unique (originally but unremarkable today) in two respects: written constitution, and not a monarchy. Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. Madison The argument against the first constitution, the Articles of Confederation, is that the federal government had essentially no power. The argument against the second, it it's effectively created a presidential democracy, or unitary executive. It's a model our diplomats expressly avoided when helping other countries write a constitution. It's fragile, and while it's so far been stable in the U.S. it's possible we've just been lucky so far. But a polyarchy doesn't guarantee democracy, even representative democracy. It certainly wasn't pluralistic at its founding: white, male, land owners. It took a long time to unwind that and here we are with most states in one form or another trying to make it harder for people to vote. Can a democracy democratically undemocratize itself? Sure. If it wants to weaken or even obsolete democracy, by any combination of action or inaction, that's what will happen. One of the classic examples is the principate period of the Roman Empire. It had the veneer of a democracy, including having a senate, but there was an emperor who had all the real authority. That period lasted over 200 years, so it's not like we'd necessarily see the end to elections, laws, casting aside the constitution, or any of the other institutions we think display proof of "democracy".