Has any (nominally) democratic government ever been domestically overthrown without military assistance?

The Articles of Confederation are probably the best example of a (set of) democratic governments overthrown by an extra-legal non military force. The Articles had (effectively) no military, so they weren't a factor.

OP adds,

When you refer to the articles of confederation, are you referring to the existing state governments being overthrown? Because "overthrow" isn't quite the impression I had there - the governments deliberately sent representatives with the intention of forming the union, so it happened with their consent. Whether or not there were specific laws on the books allowing for this procedure (I have no idea honestly) I guess seems like a mere technicality than anything else. –

I can't cite anything terse, but it is the consensus of most historians that I've read (I believe that P. Maier might have been the first to articulate it) that the Constitution was not an evolution of the articles. It is very clear that the intent of the convention was to amend the Articles, but that a small cabal (Washington, Hamilton, Jay, Madison, et. al.) usurped the Convention to advocate for a completely different government. Probably the most convincing point is that the Articles were a pact among the States, while the Constitution was very explicitly submitted to the people not the states. The signatories of the Articles were not permitted to participate in the process of selecting the new government, and directly to OP's point, while there was a procedure for amending the Articles, it was not followed. The Constitution was neither an amendment to, nor an evolution of the Articles, but a completely novel arrangement.

West Virginia probably also qualifies, as might the many governments of pre-federal Pennsylvania.

@T.E.D's example of the Protectorate seems to fit. As does the Glorious Revolution (unless you're going to exclude constitutional monarchies from democracies, which I think is ... ..I'm groping for a term, but I think that makes the definition so fuzzy and subjective as to no longer be in scope. At that point you're looking for a left handed red headed non-binary Eskimo with a nice personality and 32 cents in their left rear pocket, and if we find one, you'll argue about the coins in the 32 cents )

The first French Republic is arguable. One could argue that elements of the military contributed to the evolution from Republic to consulate, but if "elements of the military" is the condition then the question becomes nigh on impossible, since it would require either the total absence of a military or some external cause which prevented a single military person from participating.

The Russian Provisional Government probably demonstrates another edge condition. My sense is that you're looking for established democratic governments with a record of effective governance that last for at least a generation. [Aside: I get nervous when we start hedging a fuzzy definition with a set of even more fuzzy constraints]. When Karensky decided to flee the possibility of German invasion, the government was essential done. (I don't consider the German advance to be the "assistance of a foreign power", since this wasn't part of the German strategic aims)

I'd argue that any republican government ought to fit your definition - "democracy" used to be a negative term referring to mob rule, while republics had actual governance.

In Republican Rome important decisions were voted on by various assemblies. We'd argue with their franchise and their conduct of elections, but I don't see either of those constraints in the question. Rome fell to an autocracy with no external influence. Republican Rome fell several times before they gave up the ghost and the Senate.

Athens was democratic, but lost control to a tyrant. It has been too long since I read enough Athenian history to comment intelligently.

At the risk of further fuzzing a currently subjective example, I'd suggest that you restrict this to constitutional democratic or republican nation-states, which would toss out Greece & Rome, and then again find some time frame which would block the other examples. (some historians consider the American revolution to not end until the Constitution was passed). I suspect that many revolutionary governments undergo revolutions that fit your stated criteria but not your intent.

The end of the GDR mastches all of your points:

Although a firm communist part of the Warsaw pact, it called itself "German Democratic Republic" and held elections regularly. They obviously did not amnount to the "fair and free" standards of actual democracies, but in name and by fake ceremonies, they were "democratic" and a "republic".

The military was considered to be on the side of the government. While their inaction and hesitation to shed their own citizen's blood en masse had some serious influence, there was no action in favor of the "revolutionaries".

While the protestors certainly had moral support (and probably some minor logistical though any intelligence service active in East Berlin) it wasn't a planned coup by a foreign power. Or a foreign powers military supporting.