Each state has 2 senators. The one who has been in office the longest is the senior senator.
Robert Byrd.
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.
Bill Nelson (D) is the senior US Senator from Florida.
Sen. Richard J. Durbin (D-IL). He is also the current Majority Whip.
The difference is that the senior Senator has been in office longer than the junior Senator.
The current (2010) Senate Majority Leader is Harry Reid and the President Pro Tempore who by custom is the Senior Senator is Robert Byrd.
As of October 2021, Maryland's senior senator is Ben Cardin and its junior senator is Chris Van Hollen.
Alabama's senior senator is Richard Shelby
A junior senator has served in the senate for a shorter time than the other senator from his state; that other senator from his state is a senior senator.
My two Legislative representatives for Florida are senior Senator Bill Nelson and junior Senator George LeMieux.
Mary Landrieu (D-LA) is the senior US Senator from Louisiana for the term 2009-2015 and will be up for re-election in November, 2014.