The newly-elected senator from Maine was expected to do so but remained mum about his intentions during the election, when he was criticized by Republicans for not saying which party he would favor if he made it to the upper chamber.
His move gives the Democrats another pickup as he replaces retiring Republican Sen. Olympia Snowe (Maine.). Democrats will hold 55 seats in the Senate in the 113th Congress.
“The outcome of last week’s election in some ways makes this decision relatively easy. In the situation where one party has a clear majority and effectiveness is an important criteria, affiliating with the majority makes the most sense,” King said.
Read more on King's announcement on The Hill.