Final answer:
The union shop accounts for almost three-fourths of union contracts, requiring new employees to join the union after being hired. Union membership has declined significantly since its peak in 1945, coinciding with a shift from manufacturing to service industries and challenges in expanding union membership.
Step-by-step explanation:
The type of union security that accounts for almost three-fourths of union contracts is the union shop. This arrangement mandates that new hires must join the union within a certain period after they commence employment to retain their positions. This setup emerged as labor unions were established to enhance wages, working conditions, and job opportunities. However, as the United States shifted from a manufacturing-focused economy to a service-based one, union membership witnessed a significant drop. Particularly in the private sector, which now sees only around 7% of workers being union members compared to the peak of 35.5% in 1945. These dynamics reflect the evolution of job growth in manufacturing versus service sectors and the legal environment affecting union's ability to grow their membership. With unions having a stronger presence historically in manufacturing, their influence has waned as service industry jobs, including government employment, increased substantially.