The correct answer is: "the Taft-Hartley Act".
The candidates of the 1948 presidential election were, on the first hand, the incumbent President Harry S. Truman, from the Democratic party, who won more votes than republican candidate Thomas E. Dewey.
Party members from the South feared the growing weight that northern labor unions and black voters were acquiring in the party, and both were favoured when candidate Truman vetoed the Taft-Hartley Act in 1947.
The Taft-Hartley Act was a US federal law enacted in 1947 that aimed to limit the influence and actions of labor unions. Unions denominated it the "slave-labor bill". President Truman considered it a violation of the right of freedom of speech, and hence, of the main principles and civil rights on which the US society had been founded, this is why he vetoed the law on the same year of its enactment.