Federalist 39

A republican form of government is one which derives its powers either directly or indirectly from the people and is administered by persons who hold public office for a limited period of time or during good behavior. No government can be called republican that derives its power from a few people or from a favored and wealthy class. The Constitution conforms to these republican principles. The people directly elect the House of Representatives; in addition, the people indirectly select the senators and the president. Even the judges will reflect the choice of the people since the president appoints them, and the senate confirms their appointment. The president, senators, and representatives hold office for a specified and limited term; judges are appointed for life ­but subject to good behavior. The constitutional prohibition against granting titles of nobility and the guarantee to the states that they shall enjoy a republican form of government is further proof that the new government is republican in nature.