Why were many American surprise from President Johnson support civil rights?

Supporting the Civil Rights Act of 1964 was, in former U. S. President Lyndon B. Johnson's word, a risk, because he could easily lose all the support the Democratic Party enjoyed from the Southern States.

Noticeably, the white South was mostly against Title II of the aforementioned Act, which outlawed public discrimination against negros, but also people from other countries and races and people with different religious beliefs.

However, President Johnson put all his good influence among legislators to use in order to pass the bill both in Congress and Senate.