Final answer:
One-third of the U.S. Senate seats are up for election every two years in accordance with the staggered terms established for Senators, who serve six-year terms, following the rules set forth by the 17th Amendment for direct popular elections.
Step-by-step explanation:
Every two years, one-third of all Senate seats are up for election. Senators serve six-year terms, and due to the staggering of terms, approximately one-third of the Senate is elected during each midterm and each presidential election year. The constitutionally established structure ensures that the Senate is a continuous body, with only a fraction of the seats up for grabs at any given election, thus providing stability and ongoing experience within the legislative process. Following the 17th Amendment, senators are elected directly by the people rather than being chosen by state legislatures, and any vacancies that occur must be filled through a special election.