Senator Chris Dodd (D-Connecticut). Last year he was running for the White House. This year he will retire, at the end of his term. The chairman of the banking committee saw his popularity wane when the stock market collapsed and his ties to Countrywide came to light. Apparently he believes that his effort to regain his state's favor by pushing for meaningful banking regulations were getting him nowhere in the polls. Good riddance.
The Democrats didn't want his colleague from North Dakota, Senator Byron Dorgan to go. His seat will almost certainly be replaced by a Republican. Maintaining the Democratic Party's hold on Dodd's seat probably increased with his departure, however, since Dodd's numbers weren't looking so good.