From the beginning of the "I find your lack of faith disturbing" scene in "A New Hope":
The rebellion will continue to get support in the Imperial Senate.
The Imperial Senate will no longer be of any concern to us. I have just received word that the emperor has dissolved the council permanently. The last remnants of the old Republic have been swept away.
There was a twenty-odd year difference between the films right? How did the Senate stick around so long, and;
If it had any power, why didn't it use it to prevent, you know, the murder of millions of innocent people and the construction of something called a "Death Star"?
If it didn't have any power, why did it have to be dissolved at all, and why take twenty years to do it?