Independent Senator-elect Angus King of Maine said Wednesday he has decided to caucus with Democrats, which will add to the party's voting edge.
His decision ends months of speculation about which party he would align with.
The former Maine governor was elected last week to replace retiring Republican Sen. Olympia Snowe, a prominent centrist who complained about Washington's partisan gridlock in stepping down. GOP and conservative super PACs spent millions of dollars to attack King during the campaign for Snowe's seat.
With King joining their caucus, Democrats will have a 55 to 45 edge in the Senate.
King said that caucusing with Democrats will still allow him to take independent positions on issues.
"I have decided to affiliate with the Democratic Caucus because doing so will allow me to take independent positions on issues as they arise and at the same time be an effective representative for the people of Maine," King said.
Senate Democratic Leader Harry Reid of Nevada welcomed King to the caucus.
"I'm confident Senator King with be a bridge to working with Republicans," Reid said.
King said he considered not aligning with either party, but realized he could be shut out of committee assignments, where the bulk of legislative work gets done, and that would have hurt his effectiveness on issues vital to Maine.
He said aligning with Democrats does not mean he's "in automatic opposition" to the GOP.
King had requested a seat on the Senate Finance Committee, a plum assignment usually reserved for more senior lawmakers, in recent talks with Reid, but said Reid made it clear that was not likely to happen.
"My father used to say, 'If you don't ask, you don't get,'" said King. Snowe serves on the finance panel.
King has vowed to vote his conscience, saying independent voices like his are needed to try to break the partisan gridlock in Congress.
The 68-year-old King was Maine's governor for two terms between 1995 and 2003, establishing credentials as someone who could work with both parties. Before that he spent 18 years as a commentator on the Maine Public Broadcasting Network, reporting and analyzing state public policy issues.
He's a former Democrat who backs President Barack Obama, but who also supported George Bush in 2000. He backs Obama's health care overhaul. His views on issues like abortion rights and environmental causes track most closely with Democrats.
King has Capitol Hill experience, working as an aide to former Sen. William Hathaway, D-Maine, in the 1970s.