The two worked together to pass landmark campaign finance reform in 2002.
"I will ... miss the daily experience of Russ Feingold's friendship, and the qualities that distinguish his friendship – his thoughtfulness, kindness, humor and loyalty," McCain said. "I have treasured that friendship all the years we have served together, and while friendship does not end with a Senate career, I will sorely miss his presence here. I will miss seeing him every day. I will miss traveling with him. I will miss the daily reminder of what a blessing it is to have a true friend in Washington."
“Our first encounter with one another," McCain continued, "was in a Senate debate in which we argued about an aircraft carrier, somewhat heatedly, if memory serves. Russ thought the United States Navy had one too many. I thought we didn't have enough. It was, I'm sorry to admit, not a very considerate welcome on my part to a new colleague, whom I would soon have many reasons to admire. But to Russ's credit, he didn't let my discourtesy stand in the way of working together on issues where we were in agreement. And to my good fortune, he didn't let it stand in the way of our friendship either."
Feingold will be replaced by Ron Johnson, a Tea Party challenger.
McCain's full floor remarks can be found here.