Final answer:
The most effective and enduring labor union of the post-Civil War period in the U.S. was the American Federation of Labor (AFL), known for organizing skilled craft workers and promoting workers' rights such as the eight-hour workday.
Step-by-step explanation:
The most effective and most enduring labor union of the post-Civil War period was the American Federation of Labor (AFL). After the formation of the National Labor Union (NLU), which made some progress with eight-hour workdays for federal employees, the AFL rose to prominence by organizing skilled craft workers.
Under the leadership of Samuel Gompers and through strategic actions like the "no-strike pledge" during World War I, the AFL saw its membership increase significantly. The AFL's effectiveness and longevity were further demonstrated by its resilience through economic downturns, labor strikes, and eventually its merge with the Congress of Industrial Organizations to form the AFL-CIO, presently the largest union organization in the United States.
Despite challenges such as opposition to organized labor in the South and automation reducing blue-collar jobs, unions continued to grow through the 1900s, gaining rights notably during the Progressive Era and under Franklin Roosevelt's administration. The labor movement's enduring impact on workers' rights was solidified through legislation, and collective bargaining became a key component of American labor relations.