What are their principal arguments against this?

他们反对的主要理由是什么?

A principal argument for shorter hours that gained prominence during the 1930s was the notion that scarce opportunities for gainful employment ought to be shared among the largest possible number of workers.

在 1930 年代， 支持缩短工作时间的一个主要论点是认为稀缺的有酬就业机会应该由尽可能多的工人分享。