With the first session of the 114th Congress in the books several observations occur to me: (1) I’ve now worked in or around Congress for over 9% of its history – that seems like a lot to me; and (2) Congress has changed a lot in 20 years.
Over time Republicans have become less willing to cosponsor legislation and Republicans are much more alike in their willingness to cosponsor bills than Democrats. Democrats show more variety in their willingness to cosponsor bills, and on average do so more often than Republicans. Democrats have recently shown less willingness to cosponsor bills (as weighted by the number of bills outstanding) and this tendency has varied congress to congress. Since the 104th Congress Republicans have demonstrated a decreasing tendency to cosponsor bills when weighted by the number of bills outstanding.
In short: If your job is to get congressmen to support a legislative initiative—and to prove their support in an official way prior to a vote—then your job has become much harder in the last 20 years.