Answer:
They're afraid that they lost their position.
Step-by-step explanation:
The majority of Congress had to face re-election every 2-years.
At that time, the civil rights act which intended to give more rights for racial minorities were not something that popular in United States. Especially from the southern states.
Many of the congress believe that if they act in support of this act, they will lost their supports from their base (especially if that person came from southern states) and will lost their re-election. So, it was difficult for them to pass the civil rights act.