The Wagner Act contained no law governing collective agreements. Congress left their enforcement to the state and federal courts under the miserable body of common-law rules. Under various theories the courts worried about consideration, mutuality of obligation, duress and public policy aspects as if they were dealing with conventional contracts.
Charles O. Gregory,
The Law of the Collective Agreement,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol57/iss5/2