If the "they" you're referring to are the liberals, then the only outcome of this gambit, if successful, would be a more conservative nominee. Bush certainly wouldn't nominate a more liberal nominee if Roberts is rejected for being too liberal. I think you need to think your theory over.
You are complaining before the door shuts in your face, I am not aware of Roberts being in a vulnerable position. I lost access to my ISP last night and only regained it a little while ago, long enough to find a story contradicting this. Now they're kissing the asses of each side. Liberals are so far off they are making themselves the story.