It certainly looks like chicken/egg now, but not when you add a historical perspective. If you do not belive me, see what others have to say cf. Theda Skocpol _Protecting Soldiers and Mothers_ or _Social Policy in the United States_ who makes a similar argument - the fragments of working class were coopeted to the machine politics in a comparitvely open political system in the US in the late 19th and early 20th centuries and that prevented the formation of class-based politics, whereas class-wide exclusion under European system during the same time favered it.
Lets consider the following essential characteristics of the US working population: - immigrant labor entering this country in waves, each wave bringing in different nationalities - racial seggregation - the "availability" (upon extermination of the native population) of land that allowed a relative easy escape from the industrial class system - ethnic identities that differentiated working class immigrants - the I-can-be-rich-quickly mentality - the presence of machine politicians offering immediate payoffs to labor leaders willing to cooperate.
What makes you think that such an environment was not totally poisonous to the crystallization of labor-based left politics?
New Deal? I do not think most of the people who supported FDR had any change of their political orientation. This was yet another tun in the good old machine politics - they gave their vote to a politician who promised them bigger payoff, jobs, contracts, pension plans etc. Nihil novi sub solem.
Wojtek