I've searched the internet for some interview or transcript to answer this question and I can't find anything: What is Truman's argument to Senator Alben Barkley about why his veto should stand?

Answer: Truman writes that the act would adversely affect national unity and would do a disservice to the United States and the world.

Truman justified that this act would be bad for the national agenda, and that it would negatively affect the whole country.

This was the Taft-Harley act veto. This law restricts the activities and power of labor unions, they said this abused the right of the workers.