Elections are every 2 years but senators serve for 6 years so to even this out only 1/3 are elected each vote time.
1/3 of the Senators' terms expire in every 2 years, but since there are 50 states with two Senators apiece and that does not divide by 3, in two election years 34 Senators are elected (or re-elected) and in the third year the remaining 32.
About 2/3 of the 100 member population go up for election every 2 years.
Every two years, all of 435 congressional seats are up for election. In the Senate, however, their terms are staggered and approximately one- third of 100 the seats are up or election every two years since. Senators do serve for six years.
Approximately one-third of the US Senate (33 or 34 Senators) must run for reelection every two years. The situation for State Senators will vary by state.
Missouri has 34 state senators and districts. State Senators of Missouri serve four-year terms, but half of the seats are up for election every two years.
Every two years, one U.S. Senate seat of each of two thirds of the states comes up for election.
There are 2 U.S. senators from every state. John Isakson (R) and David Perdue (R) are the U.S. Senators representing Georgia. Isakson's next election will be in 2022. Perdue's next election will be in 2020.
Missouri has 34 state senators and districts. State Senators of Missouri serve four-year terms, but half of the seats are up for election every two years.
Every 6years
The Senate is elected every 6 years and the Senate is divided into classes so a third is up for election at any one time. Only the House has a 2 year term.
There are many local Election Days throughout every year and many Primary Election Days throughout every even-numbered year, but the final General Election for U. S. Senators and Representatives is always the day after the first Monday of November of even-numbered years. In leap years, that is the same day on which voters choose the electors of the U. S. President and Vice President.