There seems to have been a shift from racially motivated violence (lynchings, etc) right after the Civil War to more subtle and insidious forms of legal oppression (Jim Crowe laws, segregation). Is it accurate to say that, for some reason, violence became less socially acceptable after a period of time? If so, what was the impetus for the shift?
That would be the election of 1876.
Prior to then, the US army was physically present in the South to enforce the civil rights laws and the 14th Amendment. So the only avenue open to those wanting to maintain the (now illegal) white supremacy was terrorism.
In 1876 there was a really close election, but its pretty clear that the Republican narrowly lost it. He lot the popular vote by a good 4%. The nit is that there were laws preventing full political participation by former rebels, so the Electors for many former Confederate states were under dispute. On this basis, the Republicans had the ability to award the loser the Presidency anyway, but it would be messy.
What resulted was the Compromise of 1877. Basically, in exchange for letting the losing Republican be POTUS, the Republicans agreed to withdraw all those Federal troops enforcing the Federal civil rights laws from the entire South.
Now the white supremicsts just needed a legal fig leaf to keep the SCOTUS off their backs. This came in the 1890's with the development (in Louisiana) of the concept of "Separate but Equal", which the SCOTUS basically agreed with in 1896. After that it took the South about 5 years to fully construct the Jim Crow system. By 1900 Louisiana, a majority black state, had purged all but 5,000 of them from the voter rolls.
After this point, white supremecy had full legal sanction, so there was much less need to resort to terrorist tactics to enforce it. Of course that club was always still in the bag if needed.
No, I don't think this is correct. Violent oppression was always looked down upon by many. Moreover, lynchings were still current in the 20th century.
One cannot even say that lynchings were less common, as lynchings were actually less common e.g. c. 1875 than 1900.
According to this chart, the peak was 1892 and had significant numbers until 1904, when a downward trend is noticed, yet even then 1907 saw more lynchings than 1890.
This set of statistics, though I'm not sure how accurate it is, lists 4,946 white-on-black lynchings between 1864 and 1968. Comparing it with the other charts gives us the following numbers:
If the numbers are accurate, you can see that the number of blacks lynched actually grew worse toward the end of the 19th century before beginning to fall off around World War I and then really going down during the Great Depression and World War II.