They cant ask them to say they will vote a certain way on anything. This is unethical to make up your mind on something before its a case. Especially one that will likely come up before the court.
To wit, they made this very same objection when repubs have asked liberal appointees for a promise on an issue.
This would amount to a judge pre-judging a case. The dems would demand that judge recuse themselves because they already had made up their mond before the case ever was argued before them.