Sen. Christopher J. Dodd (Conn.), a leading Democrat whose political star fell along with the nation's economy, is expected to announce Wednesday that he has decided against running for reelection this November, according to sources briefed on the decision. Dodd, in his fifth term, chairs the Senate Banking Committee. His retirement would be the second such announcement by a Senate Democrat in 24 hours; North Dakota's Byron Dorgan said Tuesday that he would not run this fall.