Kaltenborn joined NBC in 1940. On election night in 1948, he and Bob Trout, a former CBS colleague, were at the NBC news desk to broadcast the returns of the White House race between President Harry S. Truman and challenger Thomas E. Dewey. Throughout the evening, the returns were too close to call. As the evening progressed, Kaltenborn could see a swing in Dewey's favor. It was enough for him to project Dewey the winner, although the returns were still close. What Kaltenborn did not foresee was another swing in the votes going to Truman. As evening turned to early morning, Kaltenborn retracted his original projection and announced Truman as the winner.