There were no nationwide 1939 United States elections, as only the offices of president and vice-president are elected nationwide, and these are held only every four years, on even-numbered years.
The only other federal-level elected officers are United States senator and United States representative. These are only held in even-numbered years, except to fill a vacated seat. In 1939, there were no such special elections for the Senate, and ten such elections for the House of Representatives. All ten of the newly elected representatives were of the same party as the representative who had previously occupied the vacated seat (eight Democrats, two Republicans).