If Georgia Democratic Senate candidate Jon Ossoff ultimately prevails over incumbent GOP senator David Perdue (and at this moment the issue is still in doubt) the Democrats will control the White House, the Senate, and the House of Representatives, which has occurred in five different periods since World War II.
One possible comfort for members of the GOP if they lose the Senate: in the last forty years, the two times the Democratic party has controlled all three entities it has only lasted for two years: 1993-1994, 2009-2010. In the ten years before that they controlled all three entities for four years: 1977-1980.
Following the tenure of Franklin Delano Roosevelt, when the Democrats held control of those three entities for his entire twelve-year tenure (1933-1945), the Democrats have held control between the years 1949-1952, 1961-1968, 1977-1980, 1993-1994, and 2009-2010. The GOP has controlled the three entities in 1953-54, 2003-2004, and 2016-2017.
In 1949, the Democrats held a whopping 54-42