In Episode IV, word spreads that the Emperor has just disbanded the Imperial Senate and that power would pass to regional governors. It seems a done deal at this point, as if the Emperor just had to wave his hand and poof, the Senate was automatically disbanded as a matter of law. On the other hand, much of the politics we see in the Prequels is about Palpatine's rise to power from a mostly-powerless Senator, to a Chancellor who is still subordinate to the Senate itself, and finally to a Chancellor with sufficient "emergency powers" to raise a Clone army and declare a transition to an Empire.
It seems unusual that the Senate survived almost twenty years beyond the declaration of the "First Galactic Empire", when presumably it had been a thorn in Palpatine's side since forever.
At what point did Palpatine gain the de jure legal or political authority to disband the Senate at will? Was this part of the package of emergency powers that he received in Episode II, or was this an additional power that he gained later as he coerced the Senate into making him even more powerful? If the dissolution of the Senate was not a unilateral act by the Emperor, how did the dissolution take place? Did the Senate vote itself out of existence on the "recommendation" of the Emperor? Was this a grassroots movement by Imperial citizens?
I am aware that Palpatine is a very powerful Sith darksider who is also an expert in charming and coercing people to do what he wants. I'm asking about the specific legal or political process by which the Senate was finally disbanded.