Final answer:
Around 33% of the U.S. Senate is elected every two years, during both midterm and presidential elections, as senators serve six-year staggered terms.
Step-by-step explanation:
Approximately 33% of the U.S. Senate is elected in each federal election. Senators serve six-year terms, and one-third of senators are up for election every two years during both the midterm and presidential election years. Considering there are 100 U.S. senators, this means approximately 33 senators are elected during each election cycle. This system ensures a balance between continuity and the opportunity for political change within the Senate.