The move stunned Democrats.
They were confident heading into the new year that Dorgan, a moderate Democrat in a GOP-leaning state, would run for re-election even as rumors intensified that Republican Gov. John Hoeven would challenge him in November. Early polling showed Dorgan trailing Hoeven in a hypothetical contest, and Democrats expected a competitive race if the matchup materialized.
Hoeven has not announced a candidacy but he told The Associated Press on Tuesday that he was "very seriously" considering one.
Democrats quickly started recruiting a candidate to run in Dorgan's place. Democratic Rep. Earl Pomeroy may be interested as well as Heidi Heitkamp, a former state attorney general and tax commissioner who was defeated by Hoeven in the 2000 gubernatorial race.
In Colorado, Democratic officials informed Tuesday of Ritter's decision said the governor planned to announce Wednesday that he won't run for a second term in November.
Elected in 2006, Ritter was among those Democrats who helped the party make inroads into what was once a solidly Republican state. He helped pave the way for Obama to win Colorado in 2008 and had been widely considered a rising star in the Democratic Party.
Top contenders to replace Ritter on the Democratic ticket include Denver Mayor John Hickenlooper, Interior Secretary Ken Salazar and former House Speaker Andrew Romanoff.
Two Republicans are seeking the GOP nomination: former Rep. Scott McInnis and businessman Dan Maes.
Associated Press writers David Espo, Ken Thomas and Andrew Miga in Washington, Steven K. Paulson in Denver, Dale Wetzel in Bismarck, N.D., and Susan Haigh and David Collins in Hartford, Conn., contributed to this report.