 An incoming senator who holds another office, including membership in the U.S. House of Representatives, must resign from that office before becoming a senator.A senator's seniority is primarily determined by length of continuous service; for example, a senator who has served for 12 years is more senior than one who has served for 10 years.Because several new senators usually join at the beginning of a new Congress, seniority is determined by prior federal or state government service and, if necessary, the amount of time spent in the tiebreaking office.