Sen. Chris Dodd (D-Conn.) plans to announce Wednesday that he will retire from the Senate at the end of the year, capping a 30-year career where he rose to become one of the chamber's most influential members, several Democratic sources told POLITICO Tuesday night.

Dodd's decision to retire is, at first glance, a blessing to Senate Democrats who worried they would have trouble holding the seat with the embattled senator in the race. Now Democrats expect that Connecticut Attorney General Richard Blumenthal will run in Dodd's place, giving the party a stronger nominee in a race that was widely believed to be a toss-up.

A top national Democrat said Blumenthal's candidacy was all but certain. "He's totally in," said the Democrat.

The attorney general has scheduled a 2:30 news conference on Wednesday where he's expected to announce his candidacy.

"Chris Dodd has been a great US Senator for Connecticut and the country. He's made a meaningful difference in the lives of millions of people - here in Connecticut, and across the country," said Roy Occhiogrosso, a Connecticut-based pollster who has worked for Dodd in his past races. "Attorney General Richard Blumenthal is uniquely suited to step into this campaign, win this race, and represent the people of Connecticut as we continue to confront unprecedented challenges. The Republicans thought they might steal this seat. Not anymore."

News of Dodd's plans, first reported by the Washington Post's Chris Cillizza, came on the heels of the stunning announcement Tuesday that another Democrat, Sen. Byron Dorgan, would not run for reelection in North Dakota, likely giving the GOP an advantage for his seat.

Dodd has faced months of tough press coverage surrounding his Countrywide FInancial mortgages, which followed his ill-fated decision to move his family to Iowa for his 2008 presidential run. Earlier this year, the Senate Ethics Committee cleared him of any wrongdoing related to Countrywide.

But Dodd also drew scrutiny over his role as Banking Committee chairman, where he presided over the committee's approval of billions of dollars in bonus money to AIG executives, leaving opponents to criticize him for being overly cozy with the financial industry.

The most recent Quinnipiac University poll, conducted in November, showed Dodd trailing former GOP congressman Rob Simmons by double-digits and trailing former WWE CEO Linda McMahon by a smaller margin. Even the senator's own internal polling showed him trailing Simmons - an ominous sign for a well-known incumbent.

While the Obama administration worked last year on Dodd's behalf-including fundraisers headlined by both the president and vice-president-White House aides were long convinced that the senator was the political equivalent of a dead man walking.

With poll after poll showing Dodd trailing, it became increasingly clear that his candidacy was not salvageable. A senior Democratic official said Dodd's long-rumored, and always-denied, retirement plans were "inevitable - like gravity."

Still, a source familiar with Dodd's plans insisted late Tuesday that there was no pressure exerted on Dodd from the White House to back down.

The news that Dodd would not seek re-election capped a day of Democratic retirements. Aside from Dodd and Dorgan, it was revealed that Colorado Gov. Bill Ritter would not run for re-election in 2010. And in Michigan, Lt. Gov. John Cherry announced Tuesday that he would be forgoing a run for governor.

Manu Raju contributed to this story.