To further complicate the exercise of power, elections for national office holders are staggered. Presidential elections are held every four years, Senate elections every six, and House elections every two. Because only one-third of Senate seats are contested every two years, it is impossible for all of the members of Congress and the president to be elected at the same time. The voters who select the president, the members of the House, and one-third of the Senate in one election are not likely to have the same concerns and preferences as the voters who select new House members and another third of the Senate two years later. Moreover, because the president is elected by the entire nation while members of Congress are elected by the voters in the states or districts from which they come, members of Congress and the president do not represent the same constituencies.