Your question deserves an answer at varous levels. The Scriptures teach that those who humbly confess their sin and lostness and embrace the free offer of salvation by trusting only Jesus Christ to redeem them from their sin are Christians. But that answer assumes a certain degree of mental astuteness. The Scriptures also teach that it is God who grants sinners the faith by which they embrace the Savior, thus my six year old granddaughter may truly trust and love Jesus without fully understanding all the theology behind her faith. God accepts that just as any parent would accept the simple of love his child. The Bible futher teaches that only the Lord knows our hearts, so it is a bit presumptuous for any of us to determine whether another is a Christian.
Now here's where it gets interesting and shows the manificence of the true God. God allows, even motivates, non-Christian to do his will and to perform good deeds without necessarily granting them the faith to become Christians. Jesus taught this when he pointed out that it rains on both the just and the unjust. In other words, God blesses both the believer and the unbeliever with earthly benefits. So even an athiest may do good work, discover valid principles, show a certain degree of compassion for others. His deeds may benefit humanity, but don't get him into right standing with God. The Scriptures teach that whatever is not motivated by true faith is sin. (Romans 14:23) And yet, the unbeliever's good deeds may benefit humanity. This is why you don't need to check a doctor's faith credentials before allowing him to operate on you or check a brokers's Christianity before investing.
Whether Romney is a Christian or not is not for me on anyone to judge. However, it may well be that he holds convictions and defends positions that are consistent with the Christian faith. If he is truly pro-life that would be a case in point. So in my opinion the question of Romney and of all the candidates should be, "How does his record and his commitments reflect the Judeo-Christian world and life convictions?" That we can assess, but not his heart.