George Lucas originally came up the overall story for the Star Wars prequel trilogy shortly after the first Star Wars movie hit theaters in 1977, and part of that story included the Emperor's scheme as well as the Jedi Purge. While many of these ideas shifted around, some things, such as Star Wars: Episode I - The Phantom Menace's midichlorians, eventually found their way into the Star Wars prequel trilogy. Chief among them is the rise of Sheev Palpatine as the Emperor.
The Star Wars prequels, especially Episode I, have been oft-derided over the years. However, the problem is more with execution than idea. Lucas was bogged down with his fascination of CGI, and there were his own shortcomings as a writer and director. But the through-lines of the Star Wars prequel story are interesting, and the best is how Palpatine slowly took over the Senate and built the galaxy's first Galactic Empire. And what's more interesting, that story was initially something Lucas planned way back in 1977.
According J.W. Rinzler's enormous book, The Making of Star Wars, Lucas originally imagined Palpatine's rise as the Emperor, through clever maneuvering, in 1977. Lucas' notes stated: "One of the Chancellor's began subverting the Senate and buying off the Senators with the help of the large intergalactic power companies... By the time the third term came along, he had corrupted so much of the Senate that they made him Emperor for the rest of his life." This largely fits with what happened in the Star Wars prequels. At that point, the Emperor hadn't appeared on-screen yet. Palpatine's story shifted over time, but the foundation upon which he became Emperor was always there.
Those trade laws have been the subject of much derision since The Phantom Menace released, and yet Lucas planned them to be so important over 20 years before. Prior to Palpatine's plan, the power in the Senate shifted every four years, passing from one Chancellor to the next. Palpatine set out to change this, making it so he could be re-elected. As Lucas noted, there were many people in the Senate who actually disliked the constant changing, feeling it disrupted bureaucracy, especially during a crisis (presumably referring to the Clone Wars). Palpatine was able to exploit these feelings, as Lucas noted: "They reasoned that the Emperor could bring the bureaucracy back in line. So the Emperor took control of the bureaucracy."
Again, this is the genesis of the Star Wars prequels. Darth Sidious worked from within the Trade Federation, while Palpatine manipulated Queen Amidala to calling a vote of no confidence in Chancellor Valorum. That led to Palpatine assuming the position of Chancellor, with the seeds of doubt in the position itself sown. Of course, it wasn't until Jar-Jar Binks' speech in Star Wars: Episode II - Attack of the Clones, which led to Chancellor Palpatine obtaining emergency powers, that the first real steps towards the Galactic Empire's rise were taken. There was, however, a crucial difference between Lucas' original plans and what actually happened in the oft-maligned Star Wars prequels.
In the prequel trilogy, Palpatine's rise as Emperor is completed after the execution of Order 66 in Star Wars: Episode III - Revenge of the Sith. The Jedi were almost completely wiped out and painted as rebels who conspired against the Republic. It wasn't until that so-called rebellion was quashed did Palpatine announce the formation of the Galactic Empire. And then, he became Emperor. The reverse was true in Lucas' original outline. His notes state: "There was a rebellion... many of the Senators who were fighting the Emperor at that time mysteriously died. The Jedi Knights...rallied at the Senate's side. But there was a plot afoot and when the Jedi finally rallied and tried to restore order, they were betrayed and eventually killed by Darth Vader."
This is where the gaps in Lucas' original Star Wars outline start to appear. In 1977, the Jedi Purge happened after Palpatine had become Emperor. And more importantly, it was largely done by Vader. At this point, Lucas didn't know that Darth Vader was going to be Anakin Skywalker. It makes sense, then, that he had less importance to the overarching plot. Obviously, he did then become Anakin, with his descent the focus of the Star Wars prequels. He became Darth Vader towards the end of that, but Palpatine's rise largely remained the same.