This is a complete list of United States senators during the 108th United States Congress listed by seniority from January 3, 2003, to January 3, 2005. In this congress, Jeff Bingaman (D-New Mexico) was the most senior junior senator and Mark Dayton (D-Minnesota) was the most junior senior senator
Order of service is based on the commencement of the senator's first term. Behind this is former service as a senator (only giving the senator seniority within their new incoming class), service as vice president, a House member, a cabinet secretary, or a state governor. The final factor is the population of the senator's state.
Senators who were sworn in during the middle of the two-year congressional term (up until the last senator who was not sworn in early after winning the November 2004 election) are listed at the end of the list with no number.
The most senior senators by class were Robert Byrd (D-West Virginia) from Class 1, Ted Stevens (R-Alaska) from Class 2, and Daniel Inouye (D-Hawaii) from Class 3.