With the release this week of individual Bipartisan Index scores for Senators encompassing the entire period from 1993 to 2014, we can see even more clearly the changes that have occurred within the institution.

On the GOP side, the recent partisan mindset is evident at the bottom of the list. Nine of the twelve lowest scoring Senators since 1993 belong to Republicans whose tenure began in 2005 or later.  Eight of these nine entered the Senate in 2009 or later – Tim Scott, Mike Lee, Ted Cruz, Ron Johnson, Rand Paul, Jim Risch, Deb Fischer, and Jeff Flake.

The generational change in scores among Democrats is more subtle, but no less illuminating.  Consider the Bipartisan Index scores of a "Who's Who" of older liberal icons who entered the Senate in the 1960s, 1970s, and 1980s.  Pat Moynihan, Patrick Leahy, Paul Sarbanes, Ted Kennedy, Tom Harkin, Chris Dodd, Paul Simon, John Kerry, Barbara Mikulski, Carl Levin, and Daniel Inouye all fall between +.35 and -.21 (rank 58 to 138).  Yet an ideologically comparable group of ten progressive Democrats elected since 2009 -- Al Franken, Cory Booker, Richard Blumenthal, Tammy Baldwin, Jeff Merkley, Tom Udall, Brian Schatz, Ed Markey, Chris Murphy, and Kirsten Gillibrand -- fall much lower, between -.68 and -1.14 (rank 190-219).

Few observers would say that today’s crop of Senate Democrats are more progressive than Ted Kennedy, Tom Harkin, or Barbara Mikulski.  So it is difficult to argue that the scores of those who have arrived in the Senate recently are lower because blue states have been electing Democrats with unprecedented liberal inclinations.  Yet it is undeniable that recently elected progressives are reaching across the aisle on legislation far less frequently than their iconic predecessors did a decade or two ago.

This change can also be seen in comparisons of today’s Senators with the far edges of each party in the 1990s.  Jesse Helms scored very low (-1.03719) from the 103rd to the 107th Congresses and would receive many nods as the most ideologically driven conservative of his era. But his Bipartisan Index score is higher than the eight Republicans elected since 2009 mentioned above. Similarly, Paul Wellstone, who many observers would regard as the most liberal Senator during all or most of his tenure, has a higher Bipartisan Index score (-.57) than all ten of the recently elected progressive Democrats mentioned above.

The recent spike in partisanship is evident across the ideological spectrum.  There were 66 Senators who served in one or both of the 112th and 113th Congresses and had a sample size in the Bipartisan Index of at least four Congresses.  Among those 66 Senators, 30 had the worst single Congress score of their career in either the 112th or 113th.  Only 8 had their best score in one of those two most recent Congresses.

The figures indicate that a change of political culture has taken place within the Senate in which conservatives and liberals are far less likely to engage with one another when developing and promoting legislation. These scores show changing attitudes within the Senate about the duties of a Senator and the process of legislating.  The resulting functional separation between the parties has reduced opportunities to build support for legislation at the conceptual stage that is critical to achieving a consensus.

But this doesn’t have to be.  Despite wide acknowledgement that we are living in a partisan political era, bipartisanship is possible in the Senate, even between ideological competitors.  As Senator Richard Lugar and Dean Edward Montgomery pointed out in their recent op-ed in The Hill, many recognized political stalwarts and party leaders on both sides of the aisle have scored well on the Bipartisan Index, at least some of the time.  Overall, 163 of the 227 Senators in the Index (72 percent) had a positive bipartisan score in at least one of the eleven Congresses in the study.  A great many of those 163 Senators would be considered staunchly conservative or progressive.

Often the difference between a partisan “message” bill that has no chance of passage and a bipartisan one with a focus on achieving success comes down to effort.  Genuinely attempting to pass legislation is a much more difficult, frustrating, and risky enterprise than simply staking out positions that benefit one’s political standing.  Legislating takes time, sometimes decades.  It often requires relationships to be built and political capital to be conserved.  The Bipartisan Index shows that many fewer Senators have followed this course in recent years.  But it also shows that bipartisan productivity is possible if Senators make the effort.  Voters should be demanding that they do.

Dan Diller serves as the Director of Policy at The Lugar Center.