McCain and Feingold became unlikely allies when they passed bipartisan campaign finance reform back in 2002, called the McCain-Feingold Act.
McCain acknowledged that since then things have not necessarily been as amiable:
We are of different parties and our political views are often opposed. We've had many debates on many issues. But where we agreed - on wasteful spending, ethics reform, campaign finance reform and other issues - it was a privilege to fight alongside and not against Russ Feingold.
The full text of the speech is here.