The Lincoln-Douglas debates were a series of seven debates spread across Illinois over the topic of slavery and territories. Douglas supported the Free Port Doctrine but the debates made Lincoln very popular.

http://www.illinoiscivilwar.org/debates.html Debates
http://www.mrlincolnandfreedom.org/inside.asp?ID=21&subjectID=2 Summary

Impacts of the Lincoln-Douglas debates:
1. Raised Lincolns popularity
2. Made slavery the main topic
3. Douglas won the Senate seat