Roughly, what I've heard is that they made gains under the NLRB regime of the Wagner Act but, at the price of having that be dependent on the state, which has the power to certify and decertify unions. The period of significant gains was short, lasting from the mid 30s to Taft Hartley. Then you had the post war labor compact, which lasted until the profitability crisis of the 70s made capital take away the seat they gave labor at the table to help recoup profitability.