National elections are held in the United States every two years. Each time, voters elect all members of the House of Representatives for a two-year term, and one-third of Senate members for a six-year term. Many states also choose governors and state legislatures in national elections. Citizens may also vote on different questions of state or local interest. The Constitution requires the president and vice president be elected every four years. By law, voting is to be held on the first Tuesday after the first Monday in November. Every four years, the Democrats and the Republicans hold national conventions to officially choose their nominees for president and vice president. Many states hold special primary elections to choose delegates to the national conventions. Each presidential candidate lists on state primary ballots a group of delegates who have promised to support the candidate at their party's convention. Citizens show their choice for the presidential nomination by voting for the group of delegates committed to that candidate.