Final answer:
A major population shift in the United States is the rural to urban migration, as people seek better opportunities in urban areas.
Step-by-step explanation:
The major population shift happening now in the United States is a rural to urban migration. This shift is driven by individuals moving from rural areas to urban centers in search of better employment and educational opportunities, an increase in service sector jobs, and industrial activities. Since the mid-20th century, urbanization has become a significant trend with a large portion of the population moving to metropolitan areas, particularly to suburbs. The rural-to-urban shift is a result of various long-term factors including economic development, industrialization, and climate change. This transition mirrors global migration patterns where populations move from resource-poor regions to areas with more resources and opportunities.