Nominees for the supreme court usually have already been judges and written legal opinions. A senator could say "I read the facts of [some past ruling], and I disagree with the ruling, and therefore doubt their competency to be on the supreme court."

Additionally, supreme court nominees are not selected at random--it's no accident that conservative presidents nominate conservative justices and liberal presidents nominate liberal justices. Nominees are selected by the president because the president thinks they'll like the way the nominee would rule in the future.

Qualified judges have a broad spectrum of opinions. When faced with the question, "should _this_ person have a lifetime supreme court appointment," I don't see how a senator could avoid considering how a nominee might rule in the future.