Only a strict reconstructionist could deny that.
Unfortunately for your 'argument', the Constitution doesn't say that. The guarantee is unrestricted.
You can't make it say anything else.
Walt
The guarantee is restricted to only States "in this Union". It is not an "unrestricted" guarantee at all. Those three words are a qualification of the Guarantee. You can't hallucinate your way out of that. A State can be in or out of the Union. States "in this Union" have the guarantee. States out of the Union don't have the guarantee. You can't make it say any thing about binding the States to the Union. There is nothing binding in that text. It offers an enticement to the States, nothing more. The States have the option of securing an agent, the US, to insure that they retain a Republican form of government, if they choose to be "in this Union". If a State wants some other form of government, it shouldn't ratify or remain in the Union.