While President Franklin D. Roosevelt was kept by fear of alienating the Southern powers in Congress from throwing his influence openly behind anti-lynching legislation, his Justice Department's newly created Civil Rights Division played a key role. In the early 1940s, division attorneys hit upon the idea of reviving old Reconstruction-era federal criminal statutes, which forbade conspiracies aimed at denying a citizen's civil rights and specifically those that did so in collaboration with the police.

Due to constitutional questions about these statutes' enforcement, and the perennial lack of cooperation from Southern federal judges and grand juries, efforts to use these federal laws were staggered over many years.

But the continued intercession of federal justice, and the unflagging legal and public relations efforts by the NAACP, helped to drive lynching underground. (emphasis added)

Senator Richard Russell, Democrat from Georgia, led the so-called opposition forces. The group was also known as the "southern bloc." It was composed of eighteen southern Democrats and one Republican, John Tower of Texas. Although a hopeless minority, the group exerted much influence because Senate rules virtually guaranteed unlimited debate unless it was ended by cloture. The "southern bloc" relied on the filibuster to postpone the legislation as long as possible, hoping that support for civil rights legislation throughout the country would falter. .......
The Republican Party was not so badly split as the Democrats by the civil rights issue. Only one Republican senator participated in the filibuster against the bill. In fact, since 1933, Republicans had a more positive record on civil rights than the Democrats. In the twenty-six major civil rights votes since 1933, a majority of Democrats opposed civil rights legislation in over 80 % of the votes. By contrast, the Republican majority favored civil rights in over 96 % of the votes.
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Two days later, the Senate passed the bill by a 73 to 27 roll call vote. Six Republicans and 21 Democrats held firm and voted against passage.