Mitt Romney was firm and direct with the abortion rights advocates sitting in his office nine years ago, assuring the group that if elected Massachusetts governor, he would protect the state’s abortion laws. Then, as the meeting drew to a close, the businessman offered an intriguing suggestion — that he would rise to national prominence in the Republican Party as a victor in a liberal state and could use his influence to soften the GOP’s hard-line opposition to abortion. He would be a “good voice in the party” for their cause, and his moderation on the issue would be “widely...