Spiral downward
Definition 1 of 2
Pro Tip 1/3Negative Connotation
Suggests a situation is worsening without hope of recovery. 
His grades began to spiral downward after he stopped studying.
Pro Tip 2/3Quick Descent
Implies a rapid decline, often leading to a low point. 
The team's morale spiraled downward after the loss.
Pro Tip 3/3Visual Imagery
Evokes an image of something falling uncontrollably, like a leaf in the wind. 
As rumors spread, the politician's popularity spiraled downward.