Santorum backing Spector is one mistake.
The ACU gives Santorum an 88% lifetime rating, which means that Santorum has made more than one mistake.
Your claim that the ACU ratings have "zero credibility" is not proven by your one example of Santorum backing Spector.
Did you even read my link? The ACU’s President was the one backing Spector regardless of the score differences. Show’s how much value the ACU gives to them. Santorum is irrelevant to that equation.