1) Backscratch, politically 2) Engage in vote swapping 3) Engage in vote-swapping 4) Participate in quid pro quo 5) Politick 
• (v. i. & t.) To engage in logrolling; to accomplish by logrolling.
Log'roll` intransitive verb & t. To engage in logrolling; to accomplish by logrolling. [ Political cant, U. S.]
[v] - work toward the passage of some legislation by exchanging political favors such as trading votes