Physical abuse is a high bar. You pretty much need an injury, or for the parent to lose control (e.g. drunkenness / anger management). To say any level of intensity in a spanking is too high, already puts you closer to the abolitionist position than to the US status quo.
Spanking is just low level physical abuse. There's plenty of evidence and it's widely accepted among healthcare professionals and governments (the US included) that corporal punishment of any level (spanking included) causes development problems.
Corporal punishment of children was outlawed in the US in 1969. That is two generations ago.
Here's an fairly solid statement that should suffice:
"By 2000, research was proliferating, and the convention had been ratified by 191 of the world’s 196 countries, 11 of which had prohibited all physical punishment. Today, research showing the risks associated with physical punishment is robust, the convention has been integrated into the legal and policy frameworks of many nations, and 31 countries have enacted prohibitions against the physical punishment of children.1 These three forces — research, the convention and law reform — have altered the landscape of physical punishment.
The growing weight of evidence and the recognition of children’s rights have brought us to a historical point. Physicians familiar with the research can now confidently encourage parents to adopt constructive approaches to discipline and can comfortably use their unique influence to guide other aspects of children’s healthy development. In doing so, physicians strengthen child well-being and parent–child relationships at the population level. Here, we present an analysis of the research on physical punishment spanning the past two decades to assist physicians in this important role."
