When Sen. Ben Nelson retires next year, Senate Democrats may find their party a little more unified.

That’s because the Nebraska Democrat has been the caucus’ least supportive member on party-line votes for six of the past seven years.

According to a study by CQ researchers which will be released in January, Nelson voted with his fellow Democrats only 71 percent of the time in 2011.

The next-least supportive Democrat, Sen. Joe Manchin (W.Va.), has a party unity score of 75 percent.

Among Democrats, Sens. Claire McCaskill (Mo.), Mark Pryor (Ark.) and Jon Tester (Mont.) also had low party unity scores.

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, Nelson’s 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 he’s 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, Nelson’s party unity score dropped dramatically —  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.

Nelson’s 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.