Bill Nelson, a conservative Senate Democrat
From the DC Examiner: Based on American Conservative Union (ACU) ratings (for 2007, the most recent available year), Gregg is actually among the more GOP of the RINOs, with a lifetime rating of 78.39. Lifetime ratings for other RINOs include: Maine's Olympia Snowe (49.12) and Susan Collins (52.24), and Pennsylvania's Arlen Specter (44.56),
But guess who is the most "conservative" Senate Democrat? It's Ben Nelson of Nebraska (51.73), followed by Florida's Bill Nelson (39.00), West Virginia's Robert Byrd (29.02), and Louisiana's Mary Landrieu (22.40). Overall, ACU rates Senate Democrats on average 5.90 and Senate Republicans 80.94.
In other words, if we define a Senate "moderate" regardless of party as having an ACU rating between 40 and 60, there is only one on the Democrat side, compared to four on the GOP side. There are no Democrats with scores between 60 and 79, compared to 10 GOPers (a full fourth of the caucus).
So from an ideological perspective, Democrats are clearly the party of the far left in the Senate, while the GOP represents a considerably more "diverse" portion of the ideological spectrum, with a moderately conservative average.