The study is based on 120 roll call votes in the Senate which divided mostly along party lines, with a majority of Republicans opposing a majority of Democrats.
Since he was elected in 2000, Nelsonís party unity score has ranged from a low of 36 percent in 2006 to a high of 72 percent in 2008. His 2011 score is the second highest heís had in office.
The ranking may also be a sign that Nelson was considering retiring earlier than he had indicated. In the two years before his 2006 re-election campaign, Nelsonís party unity score dropped dramatically ó perhaps he was thinking more about voters back in the red state of Nebraska than his colleagues.
His party unity score actually rose from 2010 to 2011.
Nelsonís retirement means the Republicans are likely to pick up his seat. Manchin, McCaskill and Tester all face the voters in 2012. Both McCaskill and Tester face tough races that Roll Call Politics rates as Tossups.
Sen. Ben Nelson (Neb.) has been the least supportive Democrat in the Senate in six of the past seven years, according to a study of partisan votes by CQ. Below, the percentage of time that Nelson voted with Democrats on votes that mostly split along party lines.