Up until the $1930\text{'}$s$,$ the courts defended the employment at will rule on what basis?

The constitutional basis that employers and employees both have the right to pursue liberty and property, and the government does not have the right to take that away.

The constitutional basis that employers and employees hold equal power with regards to employment and thus can negotiate fairly with each other.

The common law premise that states should control employment relationships, not the federal government.

The general idea that employees have the right to quit and change jobs without recourse so employers should have the same opportunity.