On September 13, 1788, that soon-to-expire Congress issued an ordinance giving states the authority to conduct elections for their senators and representatives. ... From 1789 to 1913, when the Seventeenth Amendment to the U.S. Constitution was ratified, senators were elected by state legislatures.
Members of the House of Representatives serve two-year terms and are considered for reelection every even year. Senators however, serve six-year terms and elections to the Senate are staggered over even years so that only about 1/3 of the Senate is up for reelection during any election.