Dorgan, a moderate who was first elected to the Senate in 1992 after serving a dozen years in the House, said he reached the decision after discussing his future with family over the holidays. Dorgan, 67, said he “began to wrestle with the question of whether making a commitment to serve in the Senate seven more years was the right thing to do.”

“Although I still have a passion for public service and enjoy my work in the Senate, I have other interests and I have other things I would like to pursue outside of public life,” he said in a statement.

Dorgan’s decision stunned members of his party, who control the Senate but are facing spirited challenges from Republicans in several states. Democrats were confident heading into the new year that Dorgan 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 national Republicans expect he will. Democrats insist they will field a strong candidate to run in Dorgan’s place, and recruitment already was underway Tuesday. Democratic Rep. Earl Pomeroy, who was first elected to the House in 1992, could be interested in seeking the Senate seat.

Dorgan’s announcement could complicate efforts by Democrats to maintain their advantage in the Senate, where they hold an effective 60-40 majority, including two independents who align themselves with Democrats. That’s just enough to break Republican filibusters if all 60 stick together.

Many Democratic incumbents could face challenges in 2010 amid high unemployment rates, concerns about the wars in Iraq and Afghanistan and anger at incumbents.

At least four Democrats, including Majority Leader Harry Reid of Nevada and five-term Connecticut Sen. Chris Dodd, are in serious trouble. Dorgan’s decision means Democrats now will have to defend open seats in three states. The others are Delaware and Illinois, where Sens. Ted Kaufman, who has Vice President Joe Biden’s old seat, and Roland Burris, who has President Barack Obama’s old seat, aren’t running for full terms.

Republicans, for their part, are defending six open seats, in Ohio, Florida, Missouri, New Hampshire, Kentucky and Kansas.

Electoral politics aside, Dorgan’s decision also could have ramifications for another of Obama’s top priorities — climate and energy legislation. With no re-election race and nothing to lose, Dorgan could be even more of a wild card on the issue than he already has been. There’s no telling how the moderate Democrat will vote if the Senate takes up the legislation this year.

Representing a large oil and coal-producing state, Dorgan opposes the bill backed by the White House and Democratic leaders that would put a limit on heat-trapping pollution and would allow companies to swap valuable emissions permits. Dorgan instead has pushed an energy bill that would boost renewable energy production and oil drilling and wait to tackle global warming pollution.

Dorgan said his decision “does not relate to any dissatisfaction that I have about serving in the Senate. Yes, I wish there was less rancor and more bipartisanship in the U.S. Senate these days. But still, it is a great privilege to serve and I have the utmost respect for all of the men and women with whom I serve.”

Dorgan is chairman of the Senate Indian Affairs Committee and leads his party’s policy committee as a member of the Senate Democratic leadership team. He has been advocate for farmers and ranchers in his home state and secured funding for renewable energy sources such as wind, solar and biofuels.