Sen.-elect Angus King, Ind-Maine, made the announcement after conferring with Sen. Susan Collins, R-Maine the previous day.
King is a former governor of the state and told reporters he thought it would be best to have one senator affiliated with the Democrats and one with the Republicans, especially since Collins is part of the minority party in the Senate.
"The outcome of last week's election in some ways makes this decision relatively easy," King said. "In a situation where one party has the clear majority and effectiveness is an important criteria, affiliating with the majority makes more sense."
King will take over the seat currently held by Sen. Olympia Snow, R-Maine, next year, tipping the slim balance in the Senate more toward the Democrats, at least unofficially.
The Portland (Maine) Press-Herald said that while King was likely to side with the Democrats on most issues, he insisted he would remain independent and not vote along party lines.