What is Truman's argument to Senator Alben Barkley about why his veto should stand?

If you are referring to the Taft-Harley act veto, then the answer should be that he considered the act to be unconstitutional because it would be breaching democratic principles. It would be limiting free speech and would limit the people's power to join into unions and fight for their labor rights. The Senate overrode the Veto.