He had the political power already on his side, with burocrats and senators under his power. The systems were under the senate control, and the senate was under his. He merely concentrated his politics to his military power. I asumed Palpatine did pretty much what Hitler did, but while Hitler blamed Germany's situation on jewish influence, Palpatine used an ongoing war created by himself and the excuse of a supposed plot of the Jedi against the republic. Palpatine's plot is the only thing I kind of apreciate on the prequels, it was quite clever, although horribly shown on the films.