Reid said Alexander doesn't get a lot of credit, but that he has played a significant role in many key decisions. As one example, Reid said the Tennessee Republican helped resolve how to handle attempts to weaken the Senate filibuster.

"He's a unique person in this body, he accomplishes a great deal, and gets credit for not a lot," Reid said. "That's unfortunate, but that's who he is and that's who he's always been."

"I know that he will continue being a stalwart in the Senate," Reid concluded.

Alexander said he would run for reelection in 2014, and will be resigning his post as Conference chairman to focus more on issues he cares about.

"Stepping down from leadership will liberate me to spend more time working for results on the issues I care most about," he said in a letter to his GOP colleagues. "I want to do more to make the Senate a more effective institution so that it can deal better with serious issues."