The Missouri Senate is the upper chamber of the Missouri General Assembly. It has 34 members, representing districts with an average population of 181,000. Its members serve four-year terms, with half the seats being up for election every two years. The Senate chooses a President Pro Tempore to serve in the absence of the lieutenant governor or when he shall have to exercise the office of governor of Missouri if there is a vacancy in that office due to death, resignation, impeachment, or incapacitation.