Republican Rep. Rick Berg has conceded the North Dakota Senate race to Democratic former state attorney general Heidi Heitkamp, completing Democrats' virtual sweep of key Senate races. 

(LM Otero/Associated Press)

Heitkamp's win means the Democrats expanded their majority by two seats on Tuesday. There will be 53 Democrats, 45 Republicans and two Independents in the next Senate. Vermont Independent Bernie Sanders has long caucused with the Democrats. Newly elected Maine Independent Angus King has not yet announced with whom he will caucus but is widely assumed to be planning to caucus with Democrats, as well, raising the Democrats effective strength to 55.

Heitkamp led Berg 50.5 percent to 49.5 percent percent, with all precincts reporting, according to The Associated Press.

Heitkamp will replace retiring Sen. Kent Conrad (D), an unlikely victory in a deeply conservative state that backed Mitt Romney over President Obama by 20 points. But Heitkamp's personal appeal and promise to work across party lines in a state that has a long tradition of ticket-splitting won the day.

Heitkamp will become the first woman senator from North Dakota when she takes office next January. A record 20 women will populate the upper chamber in the 113th Congress. 

North Dakota once looked like nearly a sure-thing pickup for the GOP. The state has grown increasingly Republican, and Obama has never been popular there. But Democrats' recruitment of Heitkamp proved to be a winning move. 

The Democrat crafted a moderate profile during the campaign, putting distance between herself and Obama on the issue of energy, which is a top issue for many voters in the state. Republicans hit back with ads that sought to tie Heitkamp to Obama on the issue of health care. But in the end, the spots were not enough to take her down.