Final answer:
The National Federation of Independent Business (NFIB) was the interest group most strongly against the passage of the Affordable Care Act, due to concerns about the impact on small businesses.
Step-by-step explanation:
The interest group strongly against the passage of the Affordable Care Act (ACA), also known as Obamacare, was the National Federation of Independent Business (NFIB). The NFIB represented small businesses and was concerned that the mandatory insurance provisions would be overly burdensome to their operations. Despite lobbying efforts from various organizations, both for and against the ACA, the act was eventually passed in 2010 with significant implications for healthcare regulation and insurance.