(*) 18. why is the senate membership elected or changed over in this way?

Senate membership is elected or changed over to ensure that the senate seats are not left vacant. This method of changing Senate membership also ensures that every state gets enough representation.

Explanation:

In general, there are two senate seats for every state legislature which means that there are two Senators from each state. The method of changing senate membership also is a purity that no bribes and corruption are involved in the selection of the senators. The US senate has 100 members who are elected for six years and one third are renewed every 2 years.