The latest issue of the Atlantic has an interesting article that considers whether or not Donald Trump can split the American labor movement: https://www.theatlantic.com/politics/archive/2017/03/can-trump-divide-organized-labor/518967/. The article regrettably does not consider the main fissure in the movement: private versus public sector unions. The current state of organized labor in the United States is comparable to how it was in the 1920s. The Republican Party controlled both houses of Congress from 1921 to 1931. Democrats took the House of Representatives in 1931, and then full control of Congress and the White House from 1933 to 1946.
Going back further, there were only two Democratic presidents from 1870 to 1933: Grover Cleveland and Woodrow Wilson. There were eleven Republicans during the same period. Some of the Republicans had very short terms; one died in office and several others only served one term. This period — which reached its apogee with the Gilded Age of the late nineteenth century — was really the formative period of American organized labour. Three major labour organization — the Knights of Labour, the American Federation of Labour, and the Industrial Workers of the World — appeared in the post-bellum US. The AFL is the only one that endured. It also adopted a pro-business and anti-radical orientation.
Since 1933, there have been eight Democratic presidents and seven Republicans. These are obviously better numbers for the Democrats that what happened in the decades after the Civil War. However, during this period, the Democrats were in control of both houses of Congress far more than the Republicans. Republicans have generally sought to weaken labour law when they have control of Congress and the White House. The next two years will be no different.
American unions are thus historically accustomed to dealing with Republican presidents, but prefer Democrats. The 1920s, when the Republican party had unified control of government, was a difficult period for American labour. It took an economic cataclysm to return the Democrats to power, which was accompanied by the rise of industrial unions. It will hopefully not require a cataclysm for the Democrats to retake Congress next year.