Robert Byrd.
Each state has 2 senators. The one who has been in office the longest is the senior senator.
Yes she is the senior Senator and Susan Collins is the junior Senator.
When Jim Webb did not run for reelection in 2012, Mark Warner, who is still in his first term (2009-2015), became the senior senator from Virginia.
Each state has two senators a Junior Senator and a Senior Senator and there is 50 states so there is 100 Senators throughout the entire United States.
Bill Nelson (D) is the senior US Senator from Florida.
Sen. Richard J. Durbin (D-IL). He is also the current Majority Whip.
The current (2010) Senate Majority Leader is Harry Reid and the President Pro Tempore who by custom is the Senior Senator is Robert Byrd.
The difference is that the senior Senator has been in office longer than the junior Senator.
Alabama's senior senator is Richard Shelby
Maryland senior senator is Sen. Barbara Mikulski. Maryland junior senator is Sen. Bejamin Cardin.
State Senator
My two Legislative representatives for Florida are senior Senator Bill Nelson and junior Senator George LeMieux.