“Voting against a pro-life amendment would permanently alienate many values oriented voters from ever voting Republican again.”
People who follow it enough to know they voted that way follow it enough to know why. This was a smokescreen, and deserved to be beaten.
Very few conditional voters "follow it enough" not to be swayed by 30 second campaign commercials. The reason that the parties and candidates spend so much on commercials is because they are so effective. Regardless of the intent, Republicans who vote against a pro-life amendment would ultimately lose the votes of many non-political class, pro-life voters.