The phrase "junior senator" refers to a person who is the least experienced or in the lower rank among the two senators representing a state in the United States. Full definition
Her uncle Robert served as junior senator from New York from 1965 until he was assassinated in 1968. (politicalticker.blogs.cnn.com)
The Democratic junior senator from New York has endeared herself to liberals in recent weeks by voting against the bulk of Trump's Cabinet picks. (observer.com)
(A spokesman for junior senator Kirsten Gillibrand said she would exercise her home state privilege and also submit a name to the White House.) (politico.com)