"Like the earlier retirements in North Dakota and Virginia, Senator Bingaman's decision immediately presents another strong pickup opportunity for Senate Republicans," said National Republican Senatorial Committee spokesperson Brian Walsh.
Bingaman's exit announcement comes amid an effort by Democratic Senate Campaign Committee Chair Patty Murray, D-Wash., and top Senate Democrats to push wavering incumbents to make quick decisions on whether to run again. When asked about that push Thursday, Murray said only that Democrats are "working hard" to field as strong a 2012 slate as possible.
With recent decisions by Webb and Bingaman, Sens. Daniel Akaka (D-Hawaii) and Ben Nelson (D-Neb.) are seen as key remaining question marks on the Democratic side.
Bingaman was a strong favorite to win re-election, but he is a reserved senator who never appeared comfortable on the campaign trail. He had reportedly been on fence for months about whether to seek another term. In a Feb. 4 interview with National Journal, Bingaman declined to say which way he was leaning but suggested he had put in enough time as a legislator.
"You know, I've been here for 28 years now," he said.
Updated at 1:53 p.m.