This is historically inaccurate. The big CIO sit-down strikes occurred before the Popular Front turn. What did happen, and this has some bearing on the recent UPS success, is that the bosses started hiring again and the workers had more leverage in waging strikes, beginning in 1934. In any case, we have been in a qualitatively different period since the end of WWII. The American Trotskyist movement always assumed that the next radicalization would be a repeat of the 1930s. SWP guru Farrell Dobbs kept seeing "heat lightning" in every single strike until the day he died, in anticipation of a repeat of the Teamsters General Strike in Minneapolis, where Dobbs became a legend.
Peter Camejo once told me that Dobbs and Jack Barnes were sitting around a swimming-pool in Los Angeles in the late 1970s, when disco was king. Dobbs told Barnes, "Jack, I don't know how long I have to live, but promise me one thing. You will return the party to its working class roots and send the petty bourgeois into the factories where the action is." Barnes, who had never had a job of any sort in his life let alone a factory job, nodded his head in agreement. Then he directed all 2000 party members to go get jobs as coal miners, steel workers, etc. Those who couldn't "make the turn" got the boot, like me. So I went to the Sandinistas and the ANC and told them I would be able to deliver leftist computer programmers wrapped up in a red ribbon. They patted me on the back and I got busy. Meanwhile, all the bonehead sectarians who got factory jobs have absolutely nothing to show for the past 20 years. Why? The working class is quiescent. When will this change? I don't know, but you won't have to squint to see it. If you do want to see effective Marxist participation in the unions today, study the Teamsters for Democratic Union in Dan LaBotz's "Rank and File Rebellion".
Louis Proyect
(http://www.panix.com/~lnp3/marxism.html)