Logrolling

This colorful phrase is often used to describe trading of votes by legislative members to obtain passage of actions of interest to each legislative member.

The phrase is likely derived from the old custom of neighbors assisting each other with the moving of logs. If two neighbors had cut a lot of timber which needed to be moved, it made more sense for them to work together to roll the logs -- if you'll help me roll my logs, I'll help you roll mine.