cast doubt
To cause someone or something to be viewed with doubt, skepticism, or uncertainty. Why did you change your mind about giving Walt the promotion? Did something happen to cast doubt?
cast doubt on (someone or something)
To cause someone or something to be viewed with doubt, skepticism, or uncertainty. I considered Walt for the promotion, but his constant lateness soon cast doubt on that decision. Such a significant error really cast doubt on all of the experiment's results.
cast doubt(s) (on someone or something)
to cause someone or something to be doubted. The police cast doubt on my story. How can they cast doubt? They haven't looked into it yet. The city council cast doubt on John and his plan.