Usurper

From the Quicksilver Metaweb.

The word usurper implies unrightful seizure of power. It was usually used to describe those who took the powers of the crown to wear it themselves. But it was also often used to describe revolutionaries, e.g. Oliver Cromwell, who hoped to set up a different order with themselves at the top.

It was also sometimes used in bitter debates in other fields, e.g. see Philosophical Transactions.

The word seems to be coming back into vogue, and has been used to describe some "winners" of questionable elections, administrators who take control of some technocratic complex by means of superior technological access or skill, and others who are accused of twisting some medium of communication and power (same thing?) to their own ends. By contrast those who use it often use such words as steward or caretaker or custodian to describe themselves as legitimate. These words also have roots in the politics of Baroque regimes, e.g. the role of regent or prime minister who did not technically rule, but in fact made all daily decisions.