How are the members of congress chosen?
The U.S. states are divided into congressional districts, the total number of which is set by Congress but cannot be less than one per state (50 since 1959) nor greater than one per 30,000 residents (10,247 in 2010). Each state's share of the total number of congressional districts must be proportional to its share of the total number of residents among the U.S. states. After every U.S. census, congressional district borders and the number of districts per state are adjusted to reflect the population shifts since the previous census. The total number of congressional districts can change at those times, too, but Congress repeatedly allows that number to remain what it was in 1913 (435) despite a 238% increase in the U.S. population between 1910 and 2010. A constitutional amendment that would make the minimum number of congressional districts one per 60,000 residents was proposed on September 28, 1789 on the same page as what we refer to as the Bill of Rights, but it has not yet received the approval of enough states for ratification.
On the day after the first Monday of November of every even-numbered year, the voters of each congressional district elect one person to be a U.S. Representative for a two-year term, and in two thirds of the states the voters of each state elect one person to be a U.S. Senator for a six-year term. So every state resident is a constituent of two U.S. Senators and one U.S. Representative. Also on those dates the voters of the District of Columbia and the five organized unincorporated U.S. territories elect Delegates to the U.S. House of Representatives for a two-year term. However, unlike the Representatives, the Delegates may vote in committee, where changes to proposals are made, but may not vote on the House floor, where final drafts of bills are accepted or rejected, and each territory/district elects one Delegate despite the size of its population. Congressional terms begin and end on the 3rd of January of odd-numbered years. Whenever a House or Senate seat becomes vacant due to death, resignation or expulsion, the applicable state Governor orders a special election to fill the seat. In the case of Senate seats, if the state allows, the Governor may appoint someone to fill the seat until the election.