How many seats in the senate are up for election every two years

Two gymnastics teams are intense rivals. the first team has started recruiting new gymnasts from different cities and even foreign countries in an effort to build a stronger team. after learning this, the second team begins its own recruitment campaign to find even better gymnasts. over a period of months, each team recruits dozens of players, each time trying to find a better recruit than its rival's most recent addition. which idea from the cold war is most similar to this scenario?