Up until this time, it was the Democrats, not the Republicans that were opposed to Civil Rights. Even the leaders opposing the Civil Rights Movement in the South were Democrats. But did Johnson predict the the Civil Rights Act of 1964 would cause the Democrats to lose the South? If so, why did he predict this?
Reportedly, according to the White House Press Secretary at the time, Bill Moyers, Lyndon B. Johnson said the night he signed the bill that Democrats "have lost the South for a generation". There's some doubt surrounding this quote but there are a variety of credible sources saying something along these lines that I have little reason to doubt that LBJ expected this to cost Democrats the South.
The reasons for this are simple: dating back hundreds of years, the South has always had more negative opinions of African Americans than the country as a whole and has generally been less approving of government regulation. An act requiring more racial integration by interfering with private enterprise and states' rights was bound to be controversial.