The key was an amendment to allow jury trials of judicial contempt proceedings, which the South wanted since it would be nearly impossible to enforce the law with all-white southern juries.
Lots of interesting things in the whole story, but an important one is how solidly labor supported the strongest civil rights position, even opposing the jury trial amendment when it was modified to apply the same jury trial requirement to unions accused of contempt, an important improvement of the situation after Taft-Hartley.
James Carey, president of the IUE, would write, rejecting an amendment to require a jury trial on civil rights contempt hearings (and now union hearings): "Labor will not barter away effective protection of the right of the Negro to register and vote" for its own gain. The AFL-CIO would echo that view, saying it would not endorse a "crippling amendment" just because it offers "advantages to organized labor."
A few unions, like the traditionally racist railroad brotherhoods and the United Mine Workers, which had suffered most from court contempt proceedings, were willing to support the jury trial amendment.
Pro-labor liberal Democrats stayed solidly against the jury trial amendment and it was mostly midwestern GOPers, never very favorable towards civil rights to begin with, who were moved to abandon the bill and seal the amendment's narrow victory.
A few liberals were moved into support of the amendment when Frank Church of Idaho added a provision to the amendment that required even unregistered blacks the right to serve on federal juries, thereby creating a new legal right for blacks and guaranteeing some racial diversity on the trials to enforce the 1957 law.
But it's a good reminder that labor unions were one of the backbones of the lobbying and struggle for civil rights in this country and even when bait was offered to trade off civil rights for their own self-interest, the AFL-CIO unions sided squarely with the strongest positions on civil rights legislation throughout the postwar period.
Nathan Newman nathan at newman.org