Palpatine was the last chancellor of the Old Republic, and the mastermind behind the biggest power-grab in galactic history. Practicing forbidden Sith techniques under his alternate identity of Darth Sidious, Palpatine arranged for his public persona to deflect suspicion while receiving praise and political respect. In time he moved from a senator representing Naboo to the Republic's Supreme Chancellor, and used the power of his office to launch the Clone Wars. With Order 66 bringing about the end of the Jedi Order, Palpatine declared himself Emperor. This marked an end to his "kindly old man" charade with which he had fooled even the wisest members of the Jedi Council.
Darth Sidious was the secret alternate identity of Palpatine. Using his Sith skills of manipulation as Sidious, he masterminded the Clone Wars and most of the events that lead to his public persona, Palpatine, becoming Supreme Chancellor and eventually Emperor.
Senator Palpatine always seemed like a political ally to Valorum. He offered what advice he could to Valorum during the troubled days of the Republic. Even when Palpatine's home planet was invaded by the Trade Federation, Palpatine was willing to defend Valorum's character, though he did admit that the Chancellor's style of leadership was too weak. It was Palpatine that quietly urged for a Vote of No Confidence that ended Valorum's rule.
When the Republic failed to offer immediate aid to Naboo during the Trade Federation invasion, Queen Amidala followed advice from Senator Palpatine and called for a Vote of No Confidence in Supreme Chancellor Valorum which lead to Palpatine becoming elected as his replacement.
Every Sith needs and apprentice and there was no better apprentice to have than the Jedi Chosen One. From their first meeting Palpatine took a special interest in Anakin Skywalker, slowly manipulating and grooming him to become his powerful and feared student, Darth Vader.
Before his rise to power, Palpatine was an unassuming yet ambitious Senator in the Galactic Republic. Palpatine saw the Republic crumbling about him, torn apart by partisan bickering and corruption. Ascending to the position of Chancellor and then appointing himself Emperor, Palpatine instituted a military build-up unprecedented in galactic history. He replaced the Galactic Republic with the New Order, a Galactic Empire that ruled by tyranny.