Obama defeated the former Massachusetts governor 56 percent to 40 percent, with 80 percent of the votes counted early Wednesday.
In winning Maine, Obama won three of the state's four electoral votes.
Maine is one of two states that apportion electoral votes by congressional district. All the other U.S. states, except Nebraska, appoint electors on a winner-take-all basis, based on the statewide popular vote.
In the race to succeed retiring Republican Sen. Olympia J. Snowe, King won 54 percent of the vote, Republican state Secretary of State Charlie Summers had 29 percent and Democratic state Sen. Cynthia Dill collected 14 percent.
King, who is socially liberal, is likely to caucus with Democrats, The Washington Post reported.
In U.S. House races, Democratic incumbents Chellie Pingree and Mike Michaud breezed to new terms.
Pingree defeated Republican Jon Courtney in Maine's 1st Congressional District, which covers the state's southern coastal area.
Michaud won against Republican Kevin Raye in the 2nd district. The 2nd district covers nearly 80 percent of the state and is the largest district east of the Mississippi River.