Final answer:
One-third of the Senate is up for election every two years, maintaining its continuous nature with staggered six-year terms.
Step-by-step explanation:
The part of the Senate that is up for election at any one time is one-third. Senate elections are staggered so that approximately one-third of the Senate is elected every two years. This ensures that the Senate is a continuous body, with a majority of its members having experience at any given time. Senators are elected for six-year terms. During midterms and presidential election years, this mechanism allows a new one-third of senators to be elected while the remaining two-thirds continue to serve.