Democratic Sens. Christopher Dodd of Connecticut and Byron Dorgan of North Dakota won't seek re-election this fall, party officials say, bringing to four the number of open Senate seats Democrats must defend to protect their majority.

Adding to the party's woes: officials said Colorado Gov. Bill Ritter planned to announce Wednesday that he won't run for a second term in November, according to two Democrats with knowledge of Ritter's decision.

The moves come at the start of an election year that's shaping up to be challenging one for lawmakers of all political stripes, and particularly for Democrats. Anti-incumbent sentiment is rippling through the electorate, a majority of the country says it' on the wrong track, and the party in power typically gets blamed for the nation's troubles.

Dodd, 66, a five-term senator whose popularity in Connecticut has tumbled since his failed 2008 presidential bid and who has faced criticism over allegations he got a favorable deal on a mortgage, was expected to disclose his decision Wednesday, according to two Democratic officials with knowledge of his plans. They spoke only on condition of anonymity ahead of Dodd's announcement.

Dorgan, a moderate who was first elected to the Senate in 1992 after serving a dozen years in the House, said Tuesday he reached the decision after discussing his future with family over the holidays. Dorgan, 67, said: "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."

Democrats face a challenging environment as they seek 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.

The decisions by Dodd and Dorgan mean Democrats now will have to defend open seats in four 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.

Dodd is chairman of Senate Banking Committee, which was at the center of efforts to deal with the economic meltdown. And he has played a prominent role in the debate over overhauling health care, taking over for his friend Sen. Ted Kennedy during his illness and then death. Given Dodd's bad poll standing, other Democrats have gone out of their way to give him the spotlight in hopes he could recover before November.