Harry Reid on Friday became the third Senate Democrat to announce plans to retire in 2016. But notably for his party's chances of regaining the majority, his retirement is the first that clearly opens the door for a Republican pickup.
Republicans meanwhile have a number of incumbents likely to seek reelection in Democratic-leaning states, meaning control of the chamber could well flip again in 2016.
Here is an early look at what the Senate battleground map looks like.
Colorado: Sen. Michael Bennet was a rare Democratic bright spot in the 2010 tea party wave, holding on to the seat he had been appointed to a...