for a party candidate in advance, electors that vote against the popular vote in their state are called faithless electors, and occurrences are rare. State law regulates how states cast their electoral college votes. In all states except Maine and Nebraska, the candidate that wins the most votes in the state receives all its electoral college votes (a "winner takes all" system). From 1969 in Maine, and from 1991 in Nebraska, two electoral votes are awarded based on the winner of the statewide election, and the rest (two in Maine, three in Nebraska) go to the highest vote-winner in each of the state's congressional districts. Congressional elections Congress has two chambers: the Senate and the House of Representatives. Senate elections The Senate has 100 members, elected for a six-year term in dual-seat constituencies (2 from each state), with one-third being renewed every two years. The group of the Senate seats that is up for election during a given year is known as a "class"; the three classes are staggered so that only one of the three groups is renewed every two years. Until the Seventeenth Amendment to the United States Constitution in 1913, states chose how to elect Senators, and they were often elected by state legislatures, not the electorate of states. House of Representatives elections The House of Representatives has 435 members, elected for a two-year term in single-seat constituencies. House of Representatives elections are held every two years on the first Tuesday after November 1 in even years. Special House elections can occur between if a member dies or resigns during a term. House elections are first-past-the-post elections that elect a Representative from each of 435 House districts that cover the United States. The non-voting delegates of Washington, D.C., and the territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico and the United States Virgin Islands are also elected. House elections occur every two years, correlated with presidential elections or halfway through a president's term. The House delegate of Puerto Rico, officially known as the resident commissioner of Puerto Rico, is elected to a four-year term, coinciding with those of the President. As the redistricting commissions of states are often partisan, districts are often drawn which benefit incumbents. An increasing trend has been for incumbents to have an overwhelming advantage in House elections, and since the 1994 election, an unusually low number of seats has changed