It's my understanding that failed attempts to pass legislation to criminalize lynching in the early-mid 20th century were numerous and repeatedly blocked or filibustered by southern Democratic senators. But how would this legislation have helped? Wasn't lynching already illegal, as the perpetrators could be charged with murder?
It was a matter of enforcement. States had anti-lynching laws but were often not enforced.
For example, Virginia passed an anti-lynching law in 1928 but it was never used to punish a white for violence against blacks (though it was used for white on white violence).
Here’s an exchange during the debate on federal anti-lynching legislation in 1948 describing the skepticism and response by Florida Congressman Bennet:
Mr. KEATING. There are cases that have been brought to Nation-wide attention where in my judgment, at least, local law enforcement officials have not taken the action which might be properly expected of them. You may disagree with that.
Mr. BENNETT. I certainly do.
Mr. KEATING. But I do believe firmly that there are such cases.
Mr. BENNETT. There may be such cases, but every one I have had a chance to look into was written with red ink, written by people who wanted to destroy the South and wanted to create fomentation between the North and the South in this country and wanted to serve the communistic cause. When you get to look at the facts, you find different situations.