Answer: Hyperbole
A hyperbole is an exaggeration that is not meant to be taken literally. It is usually used to make a persuasive point or to add emphasis. In this case, "opening a door to eternal tyranny" is an example of an exaggeration of this type. While it is likely that tyranny will not be eternal (the country itself will not be eternal), its purpose is to emphasize the risk of keeping the seat of governent vacant.