Reid said that his race in Nevada would take up campaign money that's needed much more in other competitive states for Democrats.
"We have to make sure that the Democrats take control of the Senate again," Reid said. "And I feel it is inappropriate for me to soak up all those resources on me when I could be devoting those resources to the caucus."
But he had a warning for his Republican colleagues. "My friend Senator McConnell, don't be too elated," he said. "I'm gonna be here for 22 months, and you know what I'm going to be doing? The same thing I've done since I first came to the Senate."
(RELATED: For Democrats, the Leadership Scramble Begins)
In an interview with The Washington Post Friday, Reid endorsed New York Sen. Chuck Schumer, the No. 3 Senate Democrat, to succeed him as party leader, calling him "extremely smart."
Schumer, who was considered a favorite to take over Reid's post, called Reid "one of the best humans beings I've ever met" in a glowing statement Friday. "He's so respected by our caucus for his strength, his legislative acumen, his honesty, and his determination," Schumer said. "He has left a major mark on this body, this country, and on so many who have met him, gotten to know him, and love him."
Minority Whip Dick Durbin, another contender for Reid's job, called Reid "one of the ablest leaders of the Senate Democratic caucus in modern history" in a statement Friday morning. "The Senate will miss his leadership and I will miss his friendship, but with the 114th Congress only just underway, Leader Reid and Senate Democrats have a lot of work to do on behalf of working families in this country," he said. "I will be by his side every day in that fight."
Patty Murray of Washington, the fourth-ranking Democrat and the highest-ranking woman in the Senate, praised Reid on Friday, calling him "a great leader, a true patriot, a friend, and a mentor" in a statement. "I owe so much to Harry for everything he has done, and for everything he continues to do, to help me fight for my constituents and for families across the country." Murray, who is running for reelection in 2016, could also be in line for a bump in rank in a Senate without Reid.
In a statement, President Obama called Reid "a fighter" and said, "Harry has become not only an ally, but a friend. I'm proud of all we have accomplished together, and I know the Senate will not be the same without him."
Former Secretary of State Hillary Clinton also chimed in, tweeting that she "loved serving" with Reid, a "true friend, partner [and] fighter for the middle class."
Reid's retirement gives Democrats an opportunity to introduce voters to a new candidate; likely one with higher approval ratings. In recent years, as Reid has managed the perennially stuck Senate, he has become the face of any perceived Democratic-caused gridlock. Whatever Democrat runs for the Nevada seat in 2016, they will enjoy the benefits of being on the ticket in a presidential year when higher turnout—especially of Nevada's growing Latino population—is expected to bolster Democrats' chances.