have lived in the state for at least one year, and must have lived in the district in which they are to run for at least six months, assuming the district has existed with its current boundaries for at least that long. No elected or appointed official of the United States government, including the military, may serve in the Senate, excluding those serving in the military reserves and National Guard. Similarly, no employees of the state government may serve, except for law enforcement officers, firefighters, and rescue workers. Elections and vacancies Members of the Maryland Senate are elected every four years, in off-year elections in the middle of terms for Presidents of the United States. Party nominations are determined by primary elections. The general election for Senate seats and all other state and federal elections in the normal cycle is held on the Tuesday after the first Monday in November. Should a Senate seat become vacant in the middle of a term, because of death, illness, incapacitation, disqualification, resignation, or expulsion of a member of the Senate, that seat is filled by appointment. The Central Committee of the previous Senator's party in the county or counties in which the Senate district lies makes a recommendation to the Governor on whom to appoint to the seat. Within fifteen days of the Central Committee's recommendation being selected, the Governor must appoint that person to the vacant seat. The 47 districts from which senators are elected are apportioned every ten years on the basis of population. Maryland's constitution explicitly defines the process for the drawing of these districts, requiring that the Governor make a recommendation of a new electoral map and submit it for legislative approval. As of 2005, there were approximately 112,000 people in each district. Each Senate district also elects three Delegates, and incumbent senators and delegates will often run jointly as members of incumbent slates in their districts. It is rare, however, for an incumbent to be challenged. Salaries and benefits Members of the State Senate and the House of Delegates, besides the Senate President and Speaker of the House of Delegates (referred to as the "Presiding Officers"), earn the same salary. These salaries are determined by the General Assembly Compensation Commission. After two terms, 2007-2010 and 2011-2014, during which the salary for members of the General Assembly was $43,500 per year ($56,500 for the Presiding Officers), members began receiving