It what could be viewed as either a big blow or a big opportunity, depending on your point of view, liberal Republican Sen. Olympia Snowe of Maine has announced her retirement:

“With my Spartan ancestry I am a fighter at heart; and I am well prepared for the electoral battle, so that is not the issue. However, what I have had to consider is how productive an additional term would be,” the Maine Republican said in a statement.
“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term,” she added. “So at this stage of my tenure in public service, I have concluded that I am not prepared to commit myself to an additional six years in the Senate, which is what a fourth term would entail.”

More here.