As it stands now, so long as a union acts unilaterally and in its own interest, it is immune from claims its conduct restrains trade. This was not always the case. Before the 1914 Clayton Act and the 1932 Norris Laguardia Act, "a combination of laborers to obtain a raise in wages was itself a prohibited monopoly" under the Sherman Act.
Ascribing monopoly status to trade unions did not effectively end until Norris Laguardia - passed March 23, 1932.
If you believe, as I do, that financial markets reflect what legislation will be good for the economy and which will not provide benefit, the DJIA chart for March 1932 is revealing.
The Norris LaGuardia Act, at least in concept, had been before the Congress for a number of years. One of its primary features was to prohibit a "labor injunction." On March 7, 1932, 4 were killed when police fired on 3,000 unemployed workers marching at the Ford River Rouge plant.
Mid December through March of 1932 may be characterized as consolidation, with the DJIA trading in a stable range. The last "top" in this chart pattern is March 8. The average ten moves down to the depression bottom. The market probably knew Norris Laguardia passage was a forgone conclusion within 1 1/2 days of the River Rouge incident.
Now, 80 years later we have the wreckage the 1932 market predicted. 2010 Deitroit.