Heitkamp fended off Berg to keep the seat held by the retiring Kent Conrad in the Democrats' column, CNN and MSNBC projected.
CNN said with 93 percent of the precincts tabulated, the two candidates both had roughly 50 percent of the vote, but Heitkamp had 160,752 votes to 157,758 for Berg.
Republican presidential nominee Mitt Romney and incumbent GOP Gov. Jack Dalrymple were among the winners Tuesday in North Dakota, results also showed.
Romney as expected was declared the winner of North Dakota's vote for U.S. president by CNN, though Obama won a second term with a decisive victory in the Electoral College. With 94 percent of precincts statewide reporting, Romney held a 59 percent-39 percent lead over Obama.
Meanwhile, Dalrymple -- seeking his first full term after succeeding U.S. Sen. John Hoeven in office -- beat Democratic challenger Ryan Taylor 63 percent to 34 percent.