What Is a Pro-Life Candidate?

I've been mulling over in my head the question, "What is a pro-life candidate?" On the one hand, this may seem like an easy question, but given the current choice of candidates and what they claim versus what they've done, the issue has been made much muddier. Take, for example, Mitt Romney. Romney has been on both sides of the issue and appears to be flip-flopping, much like John Kerry did on issues last election. Can someone who has flip-flopped be a pro-life candidate? Remember, Ronald Reagan at one point was pro-choice. What about Fred Thompson? His Senate vote makes him look pro-life, but his stance is that abortion is a state issue. Therefore, it makes sense that he would vote "pro-life" in Washington. Can a candidate that votes "pro-life", but leaves the decision up to the states be a pro-life candidate? What if that same candidate is pro-choice at the state level? (To be fair, I haven't found conclusive evidence of whether Fred Thompson is pro-life or pro-choice at the state level.)

On another matter (and perhaps this is a different post), why has no candidate come out as a strong pro-life candidate? It appears that all of the candidates are either vocally pro-choice or are pro-choice and paying lip service to the pro-life movement. (i.e. "I am personally pro-life, but we have to let a woman choose.")